What Is the Sound `Aqidah?
Question

What is the true belief? I would also like to know about some false beliefs.
Praise be to Allah.No deeds are accepted without sound `Aqidah
It is known on the basis of Shar`i evidence from the Quran and Sunnah that deeds and actions can only be valid and accepted by Allah if they stem from sound belief . If the belief is not correct, then the actions and words that stem from it are invalid, as Allah, may He be exalted, says (interpretation of the meaning):
{And whoever denies the faith - his work has become worthless, and he, in the Hereafter, will be among the losers.} [Al-Ma`idah 5:5]
{And it was already revealed to you and to those before you that if you should associate [anything] with Allah, your deeds would surely become worthless.}[Az-Zumar 39:65] And there are many similar verses.
What is sound `Aqidah?
The Book of Allah and the Sunnah of His Messenger (blessings and peace of Allah be upon him) clearly indicate that sound belief may be summed up as: believing in Allah, His angels, His Books, His messengers, the Last Day, and the divine decree, both good and bad. These six matters form the basis of sound belief with which the Book of Allah was sent down and with which Allah sent His Messenger Muhammad (blessings and peace of Allah be upon him).
Reference to sound `Aqidah in the Quran
There is a great deal of evidence for these six fundamentals in the Quran and Sunnah, such as the verses in which Allah, may He be glorified, says (interpretation of the meaning):

{Righteousness is not that you turn your faces toward the east or the west, but [true] righteousness is [in] one who believes in Allah , the Last Day, the angels, the Book, and the prophets ….} [Al-Baqarah 2:177]
{The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers. All of them have believed in Allah and His angels and His books and His messengers, [saying], “We make no distinction between any of His messengers”.} [Al-Baqarah 2:285]
{O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger and the Scripture which He sent down before. And whoever disbelieves in Allah, His angels, His books, His messengers, and the Last Day has certainly gone far astray.} [An-Nisa` 4:136]

Reference to sound `Aqidah in the Sunnah
There are also many authentic hadiths which point to these fundamentals, such as the famous sahih hadith which was narrated by Muslim in his Sahih from Amir al-Mu`minin `Umar ibn al-Khattab (may Allah be pleased with him), according to which Jibril (peace be upon him) asked the Prophet (blessings and peace of Allah be upon him) about faith , and he said to him: “Faith is to believe in Allah, His angels, His Books, His messengers, and the Last Day, and to believe in the divine decree, both good and bad…” This hadith was also narrated by Al-Bukhari and Muslim from Abu Hurayrah.
From these six fundamentals stem everything that the Muslim must believe in with regard to Allah, may He be glorified, and with regard to the Resurrection, and other matters of the unseen.
Fundamentals of Sound `Aqidah
Belief in Allah
Belief in Allah, may He be glorified, includes believing that He is the [only] true God Who is deserving of worship, to the exclusion of all others, because He is the Creator of people, the One Who cares for them, the One Who provides for them, the One Who has knowledge of all that they do, both in secret and openly, the One Who is able to reward those who obey Him and punish those who disobey Him. It is for this purpose of worshipping Him that Allah created the two races [of humankind and the jinn] and commanded them [to worship Him Alone].
The true meaning of this worship is to single out Allah, may He be glorified, Alone for all acts of worship that people do, such as calling upon Him in supplication, fear, hope, prayer, fasting, offering sacrifices, making vows and other types of worship, which are to be done for Him Alone, with humility, seeking His pleasure and fearing His wrath, with perfect love for Him, humbling themselves before His might.
Another part of believing in Allah is believing in everything that He has enjoined upon His slaves and made obligatory for them, such as the five pillars of Islam – which are: testifying that there is no god worthy of worship except Allah, and that Muhammad is the Messenger of Allah; establishing prayer; giving zakah; fasting Ramadan; and going on pilgrimage to the Sacred House of Allah (Hajj), for the one who is able to do that – and other obligatory deeds which are mentioned in Islamic teachings.
The greatest and most important of these pillars is testifying that there is no god worthy of worship except Allah and that Muhammad is the Messenger of Allah. Testifying that there is no god worthy of worship except Allah dictates that one should devote worship sincerely to Allah Alone, and refrain from worshipping anyone or anything besides Him. This is the meaning of La ilaha illa Allah (there is no god worthy of worship except Allah), for what it means is that there is none that is deserving of worship except Allah, so everything that is worshipped besides Allah, whether a human, an angel, a jinni or anything else, is worshipped on the basis of falsehood, and the only one Who is worshipped on the basis of truth is Allah Alone, as He, may He be glorified, says (interpretation of the meaning):
{That is because Allah is the Truth, and that which they call upon other than Him is falsehood.} [Al-Hajj 22:62]
Another part of believing in Allah is believing in His beautiful names and sublime attributes, as narrated in His Book and in sound reports from His Messenger (peace and blessings be upon him), without distorting or denying the meaning, without discussing how they are, and without likening Him to His creation. Rather they must be taken as they are, without discussing how, whilst believing in what is indicated by the sublime attributes that are ascribed to Allah, may He be glorified and exalted. We must affirm these attributes in the manner that is befitting to Him, without likening Him to His creation in any of His attributes, as Allah, may He be glorified, says (interpretation of the meaning):
{There is nothing like unto Him, and He is the Hearing, the Seeing.} [Ash-Shura 42:11]
Belief in the Angels
Belief in the angels includes believing in them in general terms and in specific terms. So the Muslim believes that Allah has angels whom He created to obey Him, and He has described them as honoured slaves who do not speak until He has spoken and they carry out His commands:
{He knows what is [presently] before them and what will be after them, and they cannot intercede except on behalf of one whom He approves. And they are filled with reverent awe of Him.} [Al-Anbiya` 21:28]
Angels in Islam are of many kinds. Some of them are appointed to be the bearers of the Throne; some of them are the keepers of Paradise and Hell; some of them are appointed to keep a record of people’s deeds.
And we believe in specific terms in those whom Allah and His Messenger (peace and blessings be upon him) have mentioned by name, such as Jibril, Mika`il, Malik the keeper of Hell, and Israfil who is appointed to blow the Trumpet [on the Day of Resurrection]; he is mentioned in sound hadiths. It is proven in As-Sahih from `Aishah (may Allah be pleased with her) that the Prophet (blessings and peace of Allah be upon him) said: “The angels were created from light, the jinn were created from smokeless fire, and Adam was created from what has been described to you.” (Narrated by Muslim in his Sahih)
Belief in the Books of Allah
It is obligatory to believe in general terms that Allah, may He be glorified, sent down Books to His prophets and messengers to explain the truth and call people to it, as Allah, may He be exalted, says (interpretation of the meaning):
{We have already sent Our messengers with clear evidences and sent down with them the Scripture and the balance that the people may maintain [their affairs] in justice.} [Al-Hadid 57:25]
We believe in specific terms in those which Allah has mentioned by name, such as the Tawrah (Torah), the Injil (Gospel), the Zabur (Psalms) and the Quran.
The Noble Quran is the best of them and the last of them, and it supersedes them and confirms them. It is the one which all people are obliged to follow and refer to for judgement, along with what is soundly narrated from the Messenger of Allah (blessings and peace of Allah be upon him) of the Sunnah, because Allah, may He be glorified, sent His Messenger Muhammad (blessings and peace of Allah be upon him) as a messenger to both races [humankind and the jinn], and He sent down to him this Quran so that he might judge between them according to it. He has made it a healing for what is in their hearts, and an explanation for all things, guidance and mercy for the believers, as Allah, may He be exalted, says (interpretation of the meaning):
{And this [Quran] is a Book We have revealed [which is] blessed, so follow it and fear Allah that you may receive mercy.} [Al-An`am 6:155]
Belief in the Messengers of Allah
It is obligatory to believe in the messengers in general terms and in specific terms. Hence we believe that Allah, may He be exalted, sent to His slaves messengers from among them, as bringers of glad tidings and warners, and as callers to the truth. Whoever responds to their call will attain bliss, and whoever goes against them will incur loss and regret. The last and best of the messengers is our Prophet Muhammad ibn `Abdillah (blessings and peace of Allah be upon him), as Allah, may He be glorified, says (interpretation of the meaning):
{And We certainly sent into every nation a messenger, [saying], “Worship Allah and shun false gods”.} [An-Nahl 16:36]
With regard to those whom Allah has mentioned by name, or their names are soundly narrated from the Messenger of Allah (peace and blessings be upon him), we believe in them in specific terms, such as Nuh, Hud, Salih, Ibrahim and others – blessings and peace of Allah be upon them and upon our Prophet.
Belief in the Last Day 
Belief in the Last Day includes believing in everything that Allah and His Messenger (blessings and peace of Allah be upon him) have told us about what will happen after death, such as the trial and punishment or bliss in the grave, and what will happen on the Day of Resurrection of terrors, hardships, the Sirat, the balance, the reckoning, requital, and the distribution of the records among the people, when some will receive their record in their right hands and others will receive their records in their left hands or behind their backs. That also includes believing in the cistern of our Prophet Muhammad (blessings and peace of Allah be upon him) to which people will come, and believing in Paradise and Hell, and believing that the believers will see their Lord, may He be glorified, and He will speak to them, and other things that are mentioned in the Quran and soundly narrated in the authentic Sunnah from the Messenger of Allah (blessings and peace of Allah be upon him). We must believe in all of that and affirm it in the manner in which Allah and His Messenger (blessings and peace of Allah be upon him) have explained it.
Belief in the divine decree 
Belief in the divine decree includes believing in four things: Allah`s knowledge, His writing all things in Al-Lawh Al-Mahfuz, creation and the divine will . 
Important aspects of belief in Allah

Believing in Allah also includes believing that faith consists of words and deeds, and it increases by doing righteous deeds and decreases by committing sins; and that it is not permissible to deem any Muslim to be a disbeliever for committing any sin that is less than shirk or disbelief, such as zina, theft, consuming riba, consuming intoxicants, defiantly disobeying parents, and other major sins, so long as he does not regard those things as permissible, because Allah says:

{Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills.} [An-Nisa` 4:48]
And it is proven in the Mutawatir Hadiths narrated from the Messenger of Allah (blessings and peace of Allah be upon him) that Allah will bring forth from the Fire anyone who has in his heart faith the size of a mustard seed.

Part of believing in Allah is loving for the sake of Allah, hating for the sake of Allah, taking some people as allies for the sake of Allah, and taking others as enemies for the sake of Allah. So the believer loves the believers and regards them as allies, and he hates the disbelievers and regards them as enemies.

Foremost among the believers of this ummah are the Companions of the Messenger of Allah (blessings and peace of Allah be upon him). Ahl as-Sunnah wal Jama`ah love them and regard them as allies, and they believe that they are the best of people after the prophets, because the Prophet (blessings and peace of Allah be upon him) said: “The best of people are my generation, then those who come after them, then those who come after them.” Sahih – agreed upon.
They believe that the best of them are: Abu Bakr as-Siddiq, then `Umar al-Faruq, then `Uthman Dhu`n-Nurayn, then `Ali al-Murtada (may Allah be pleased with them all). After them come the rest of the ten who were given the glad tidings of Paradise, then the rest of the Companions (may Allah be pleased with them all). 
They refrain from discussing the disputes that arose among the Companions, and they believe that in that case each of them was striving to reach the right conclusion and make the right decision; whoever got it right will have two rewards and whoever got it wrong will have one reward. 
They love the family members of the Messenger of Allah (blessings and peace of Allah be upon him) who believed in him, and they regard them as allies. They regard as allies the wives of the Messenger of Allah (blessings and peace of Allah be upon them), the Mothers of the Believers, and they pray that Allah be pleased with all of them.
They reject the way of the Rafidis who hate and revile the Companions of the Messenger of Allah (blessings and peace of Allah be upon him), and exaggerate about the members of the Prophet`s family (Ahl al-Bayt), raising them to a status higher than that which Allah, may He be glorified and exalted, bestowed upon them. They also reject the way of the Nasibis who revile Ahl al-Bayt in words or deeds.
Everything that we have mentioned above is the sound `Aqidah (belief or creed) with which Allah sent His Messenger Muhammad (blessings and peace of Allah be upon him). It is the creed of the saved group, Ahl as-Sunnah wal Jama`ah , concerning which the Prophet (blessings and peace of Allah be upon him) said: “A group of my ummah will continue to adhere to the path of truth and they will continue to prevail, and they will not be harmed by those who forsake them, until the decree of Allah comes to pass.” And he (blessings and peace of Allah be upon him) said: “The Jews split into seventy-one groups, and the Christians split into seventy-two groups. This ummah will split into seventy-three groups, all of whom will be in the Fire except one.” The Companions said: Who are they, O Messenger of Allah? He said: “Those who follow that which I and my Companions follow.” This is the `aqidah and sound creed to which we must adhere and remain steadfast therein, and we must beware of anything that goes against it.
False beliefs from an Islamic perspective

As for those who deviate from this `aqidah and follow the opposite, they are of many kinds. Among them are those who worship idols , images, angels, the awliya’ (“saints”; close friends of Allah), the jinn, trees, rocks and so on. 

Those people have not responded to the call of the messengers; rather they have gone against them and stubbornly rejected them, as Quraysh and other Arabs did with our Prophet Muhammad (blessings and peace of Allah be upon him); they used to ask their objects of worship to meet their needs, heal the sick and grant them victory over their enemies, and they used to offer sacrifices to them and make vows to them. When the Messenger of Allah (blessings and peace of Allah be upon him) rebuked them for that, and instructed them to devote worship sincerely to Allah Alone, they found that odd and rejected it, and they said:
{“Has he made the gods [only] one God? Indeed, this is a curious thing.”} [Sad 38:5]
Then circumstances changed, and ignorance prevailed over most people until the majority have gone back to the religion of the Jahiliyyah, exaggerating about the prophets and the awliya’, calling upon them, seeking their help and other kinds of shirk (associating others with Allah). They do not understand the true meaning of La ilaha illa Allah (there is no god worthy of worship except Allah) as even the disbelievers among the Arabs understood it. This shirk continued to spread among the people until the present day, because of the prevalence of ignorance, and because we are so far from the era of prophethood.

One of the kinds of creeds based on disbelief that are opposed to sound `aqidah and opposed to the message of the messengers (blessings and peace of Allah be upon them) is what the atheists believe in nowadays, namely the followers of Marx, Lenin and others who promote atheism and disbelief, regardless of whether they call that socialism or communism or Baathism or any other names. One of the basic principles of these atheists is that there is no God and life is purely materialistic. Other basic principles of theirs include denial of the Resurrection, denial of Paradise and Hell, and disbelief in all religions. Anyone who examines and studies their books will realise that for sure. Undoubtedly this creed is opposed to all the divinely-revealed religions and will lead its followers to the worst consequences in this world and the Hereafter.
Another kind of creed that is opposed to the truth is what some esoterics (batinis) and some Sufis believe in, that some of those whom they call awliya’ (“saints” or “holy men”) share control with Allah, directing the affairs of the universe. They call them by titles such as aqtab, awtad, aghwath and other names that they have fabricated for their gods. This is one of the most reprehensible types of associating others with Allah in His Lordship, and it is worse than the shirk of the Arabian Jahiliyyah, because the disbelieving Arabs did not associate others with Allah in His Lordship; rather they associated others with Him in their worship, and they did that at times of ease, but at times of hardship, they devoted their worship sincerely to Allah Alone, as Allah, may He be glorified, says (interpretation of the meaning):

{And when they board a ship, they supplicate Allah, sincere to Him in religion. But when He delivers them to the land, at once they associate others with Him.} [Al-`Ankabut 29:65]
With regard to divine Lordship, they acknowledged that it belongs to Allah Alone, as He, may He be glorified, says (interpretation of the meaning):
{And if you asked them who created them, they would surely say, “Allah”.} [Az-Zukhruf 43:87]
{Say, “Who provides for you from the heaven and the earth? Or who controls hearing and sight and who brings the living out of the dead and brings the dead out of the living and who arranges [every] matter?” They will say, “Allah,” so say, “Then will you not fear Him?”} [Yunus 10:31] And there are many similar verses.

Among the creeds that are opposed to sound Islamic `aqidah regarding the divine names and attributes are the beliefs of the innovators such as the Jahmis, Mu`tazilah and others who followed their way in denying the attributes of Allah, may He be glorified and exalted, and denying His attributes of perfection, and they ascribed to Him, may He be glorified and exalted, attributes of non-existent beings, inanimate beings and impossible beings – exalted be Allah far above what they say.
That also includes those who deny some of the divine attributes and affirm others, such as the Ash`aris , who should have applied the same argument as they applied to the attributes that they affirmed to the attributes that they denied and interpreted in a manner other than what was indicated by the apparent meaning. Thus they went against both the textual evidence and the rational evidence, and contradicted themselves.

Source: 
Summarised from the essay Al-`Aqidah As-Sahihah wa ma Yudadduha by Shaykh `Abd Al-`Aziz ibn Baz (may Allah have mercy on him).
And Allah knows best.