Which is worse: minor shirk, major sins or innovation (bid‘ah)?
Question

I hope that you can put these five prohibited matters in order, starting with the most serious: minor shirk, major sins, innovations, major shirk and minor sins.
I know that the most serious of them is major shirk and the least serious is minor sins, but I do not know in what order the others should appear in between these two.
Praise be to Allah.Undoubtedly the matter is as the questioner said, which is that the most serious of all prohibited matters is falling into major shirk, which is the sin that renders all good deeds null and void, and which Allah, may He be exalted, does not forgive unless one repents from it, although He may forgive lesser sins for whomever He will. Allah, may He be glorified and exalted, says (interpretation of the meaning):
“Verily, Allah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, and whoever sets up partners with Allah in worship, he has indeed invented a tremendous sin” [an-Nisa’ 4:48]
“And indeed it has been revealed to you (O Muhammad (blessings and peace of Allah be upon him)), as it was to those (Allah’s Messengers) before you: ‘If you join others in worship with Allah, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers’” [az-Zumar 39:65]. 
Although major shirk is the most serious of prohibited actions and sins, it comes under the heading of major sins, although it is the gravest and most serious of them all. 
It was narrated that ‘Abdullah ibn Mas‘ood (may Allah be pleased with him) said: A man said: O Messenger of Allah, which sin is most grievous before Allah? He said: “Attributing an equal to Allah when He has created you.”
 Narrated by al-Bukhaari (4477) and Muslim (86). 
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Avoid the seven sins that doom a person to Hell.” They said: What are they, O Messenger of Allah? He said: “Associating others with Allah (shirk); witchcraft; killing a soul whom Allah has forbidden us to kill, except in cases dictated by Islamic law; consuming riba; consuming orphans’ wealth; fleeing from the battlefield; and slandering chaste, innocent women.”
Narrated by al-Bukhaari (6857) and Muslim (89). 
With regard to innovations (bid‘ah), not all of them are of the same degree of seriousness. Innovations are divided into categories according to the rulings on them and their consequences; those categories are innovations that constitute disbelief (kufr) and innovations that do not constitute disbelief. 
Shaykh Haafiz Hakami said in Ma‘aarij al-Qubool (3/1026): 
Moreover innovations are divided into two categories according to their impact on one’s faith: those which constitute disbelief to the one who engages in them and those which do not constitute disbelief. 
The guideline on innovation that constitute disbelief is that a person denies a command on which there is scholarly consensus, it is narrated via mutawaatir reports, and it is so well established that no Muslim has any excuse for not knowing it, such as denying something that is obligatory, regarding as obligatory something that was not enjoined as obligatory, or regarding as permissible something that is prohibited, or regarding as prohibited something that is permissible, or believing something that is not befitting to Allah and His Messenger and His Book, whether that is denying or affirming something, because that constitutes disbelief in the Book and in that with which Allah sent His Messenger (blessings and peace of Allah be upon him – such as the innovation of the Jahamis who denied the divine attributes and said that the Qur’an was created, or saying that any of the attributes of Allah is created, or denying that Allah, may He be exalted, took Ibraaheem as a close friend, or that He spoke directly to Moosaa, and so on; or the innovation of the Qadaris who denied the knowledge and deeds of Allah, may He be glorified and exalted, and denied His will and decree; or like the innovation of the Mujassimah who likened Allah, may He be exalted, to His creation; and other ideas that are based on whims and desires. 
But among these people were some who knew what they were doing and their aim was to undermine the basic principles of the religion and cause its people to doubt it. Such a person is definitely a disbeliever; in fact he is a complete stranger to the faith and one of the most hostile of enemies to it. 
However others are known to have been deceived and got confused; in their case they are only deemed to be disbelievers after binding proof has been established against them. 
The second category is innovation that does not constitute disbelief. This is that which does not imply rejection of the Qur’an or of any of that with which Allah sent His Messenger, such as the innovation of the Marwaanis which was denounced by the senior Sahaabah, who never approved of it, but they did not deem them to be disbelievers because of that, and they did not recant their allegiance to them because of that, such as when they delayed some of the prayers until the end of their times, or they delivered the khutbah before the Eid prayer, or they sat whilst delivering the khutbah on Fridays and other occasions, and they reviled the senior Sahaabah from the minbar, and other things they did, not because they thought it was based on Islamic teachings; rather they based them on misinterpretations, whims and desires, and pursuit of worldly gains. End quote. 
Similarly, with regard to innovations, some of them constitute major sins and some constitute minor sins. The guideline on differentiating between them is the extent to which the innovation is contrary to the basic teachings of Islam. 
Ash-Shaatibi said in al-I‘tisaam (2/540): 
Major sins are limited to the sins that undermine the basic principles in every religion, namely: religious commitment, life, offspring, reason and wealth. Every text deals with these issues, and any text that does not refer to these principles comes under the same categories. That includes all issues that the scholars discussed or did not discuss that come under the same categories. 
Similarly we say with regard to major innovations: any innovation that undermines one of these basic principles is a major sin, and whatever is not like that is a minor sin. End quote. 
With regard to minor shirk, although it comes under the general heading of major sins, it is regarded, because it is an issue of ‘aqeedah (belief), as being more serious than major sins that consist of actions that do not have to do with ‘aqeedah. 
It was narrated from a number of the early generations that they stated that minor shirk was more serious than major sins, quoting as evidence for that the words of the Messenger of Allah (blessings and peace of Allah be upon him): “What I fear most for you is minor shirk.” They said: O Messenger of Allah, what is minor shirk? He said: “Showing off. For Allah, may He be blessed and exalted, will say on the Day when He will requite all people for their deeds: ‘Go to those for whom you used to show off with your deeds in the previous world, and see whether you find any reward with them.’”
Narrated by Ahmad (27742); classed as saheeh by al-Albaani in Saheeh al-Jaami‘ (1555). 
Similarly, ‘Abdullah ibn Mas‘ood (may Allah be pleased with him) said: To swear by Allah even when telling a lie is dearer to me than swearing by anything other than Him even if I am saying something true.
Narrated by at-Tirmidhi in at-Tarheeb wa’t-Targheeb (4/58); classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (2562) 
This hadith may indicate that this specific type of minor shirk, swearing by something other than Allah, is more serious than the particular sin mentioned along with it, namely swearing a false oath. That does not mean that everything that is classified as minor shirk is worse than everything that is classified as a major sin. Rather, as we have said, that is a general rule, but it is not applicable in all cases, as there are some grave major sins that are far worse than minor shirk. 
Shaykh ‘Abd ar-Rahmaan al-Barraak (may Allah preserve him) was asked: 
Is minor shirk worse than major sins, and does this apply in all cases? 
He replied: 
The texts indicate that shirk may be major or minor. 
Major shirk is contrary to the basic principles of faith and Tawheed, and implies that one is an apostate from Islam, will abide for eternity in Hell, and all one’s good deeds will be rendered null and void. The correct view is that it is the sin that will not be forgiven, as Allah, may He be exalted, says (interpretation of the meaning): “Verily, Allah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases” [an-Nisa’ 4:48]. 
With regard to minor shirk, it is not like that; it comes under the heading of sins that are less serious than major shirk, so it is included in the general meaning of the words: “but He forgives except that (anything else) to whom He pleases”. 
It is of various types: 
Shirk in the heart, such as showing off, which is mentioned in the hadith of the Prophet (blessings and peace of Allah be upon him): “What I fear most for you is minor shirk.” He was asked about that and he said: “Showing off. 
Another type has to do with words that are spoken, such as swearing by anything other than Allah, as the Prophet (blessings and peace of Allah be upon him) said: “Whoever swears by anything other than Allah has committed shirk (associated something with Allah).” 
Another example is if a man says: “Were it not for Allah and you”, or “This is from Allah and you”, or “Were it not for the little dog the thief would have come to us”, “Were it not for the goose in the house, the thief would have come to us”, as it says in the report narrated from Ibn ‘Abbaas in the commentary on the verse in which Allah, may He be exalted, says (interpretation of the meaning): “Then do not set up rivals unto Allah (in worship) while you know (that He Alone has the right to be worshipped)” [al-Baqarah 2:22]. 
Another example is if a man says: “Whatever Allah wills and you will.” 
Some of the scholars stated that in the view of the early generations, minor shirk was more serious than major sins. This is supported by the words of Ibn Mas‘ood (may Allah be pleased with him): To swear by Allah even when telling a lie is dearer to me than swearing by anything other than Him even if I am saying something true. 
It is well-known that swearing by Allah falsely comes under the heading of yamen ghamoos (a false oath that could cause a person to enter Hell), yet despite that he thought that this was less serious than swearing by something other than Allah. 
What appears to be the case – and Allah knows best – is that instances of minor shirk are not all of the same degree; rather some of them are more sinful and haraam than others. 
Swearing falsely by Allah is more serious than a man saying “Whatever Allah wills and you will”,  because it is narrated in the hadith of at-Tufayl, that was narrated by Ahmad and others, that the Sahaabah (may Allah be pleased with them) used to say: Whatever Allah wills and Muhammad wills”, and the Prophet (blessings and peace of Allah be upon him) did not tell them not to do that in the beginning, until at-Tufayl saw the dream and told the Prophet (blessings and peace of Allah be upon him) about it, then the Prophet (blessings and peace of Allah be upon him) addressed them and told them not to do that, and he said: “You used to say something that shyness before you prevented me from telling you not to do it; do not say, ‘Whatever Allah wills and Muhammad wills.’” 
According to another report: “Say: ‘Whatever Allah wills then whatever Muhammad wills.’” 
Classed as saheeh by the commentators on al-Musnad (no. 20694) 
What appears to be the case is that the view of the early generations, that minor shirk is more serious than major sins, is applicable to anything that comes under the same category (namely shirk), such as such as swearing oaths, because swearing by something other than Allah is more serious than swearing falsely by Allah, as it says in the report of Ibn Mas‘ood. The category of shirk is more serious than the category of major sins, but that does not imply that everything that is described as minor shirk is more serious than everything that is described as being a major sin, because some major sins are spoken of in the sternest terms and there is the strictest warning against them, such as is not the case with regard to some types of minor shirk, as we see in the example given above about a man saying “Whatever Allah wills and you will.” And Allah knows best.
End quote. 
And Allah knows best.