How to Give Da’wah (Calling Others to Islam)
Question

How should we call people to Islam?
Praise be to Allah.Importance of following the path of Allah 
Allah created man and caused him to dwell on this earth, but He did not forsake him. Rather He gave him what he needs of food, drink and clothing, and revealed to him throughout the ages a path to follow and be guided by. The interests and happiness of mankind at every time and in every place depend upon following the path of Allah and rejecting all others:
“And verily, this (i.e. Allah's Commandments mentioned in the above two Verses 151 and 152) is My straight path, so follow it, and follow not (other) paths, for they will separate you away from His path. This He has ordained for you that you may become Al-Muttaqun (the pious)” [al-An'am 6:153 – interpretation of the meaning]
Islam is the last of the heavenly religions and the Quran is the last of the heavenly Books. Muhammad (peace and blessings of Allah be upon him) is the last of the Prophets and Messengers. Allah commanded him to convey this religion to all of mankind:
“this Quran has been revealed to me that I may therewith warn you and whomsoever it may reach.” [al-An’am 6:18]
Allah sent His Messenger Muhammad (peace and blessings of Allah be upon him) with Islam to all of mankind, as Allah says (interpretation of the meaning):
“Say (O Muhammad): ‘O mankind! Verily, I am sent to you all as the Messenger of Allah'”[al-A'raf 7:158]
Da’wah is the best of deeds
Da’wah or calling others to Islam is the best of deeds, because it involves guiding people to the Straight Path and to that which will bring them happiness in this world and in the Hereafter:
“And who is better in speech than he who [says: ‘My Lord is Allah (believes in His Oneness),’ and then stands firm (acts upon His Order), and] invites (men) to Allah’s (Islamic Monotheism), and does righteous deeds, and says: ‘I am one of the Muslims.’”[Fussilat 41:33 – interpretation of the meaning]
Da’wah: The mission of prophets and messengers
Calling others to Islam is an honourable mission. This is the work of the prophets and messengers. The Messenger (peace and blessings of Allah be upon him) explained that his mission in life, and the mission of his followers, is to call people to Allah . Allah says (interpretation of the meaning):
“Say (O Muhammad): ‘This is my way; I invite unto Allah (i.e. to the Oneness of Allah — Islamic Monotheism) with sure knowledge , I and whosoever follows me (also must invite others to Allah, i.e. to the Oneness of Allah — Islamic Monotheism with sure knowledge). And Glorified and Exalted be Allah (above all that they associate as partners with Him). And I am not of the Mushrikun (polytheists, pagans, idolaters and disbelievers in the Oneness of Allah; those who worship others along with Allah or set up rivals or partners to Allah)’.”[Yusuf 12:108]
Muslims are commanded to call people to Islam
The Muslims in general and the scholars in particular are commanded to call people to Islam , as Allah says (interpretation of the meaning):
“Let there arise out of you a group of people inviting to all that is good (Islam), enjoining Al-Ma‘ruf (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden). And it is they who are the successful'' [Aal 'Imran 3:104]
The Prophet (peace and blessings of Allah be upon him) said: “Convey from me even if it is (only) one ayah.” (Narrated by al-Bukhari, 3461)
What is da’wah?
Calling people to Allah is an important task and a glorious mission, because it means calling people to worship Allah Alone. It means bringing them forth from darkness to the light, planting goodness in the place of evil and truth in the place of falsehood. 
Conditions of da’wah
Hence whoever does this needs to have knowledge, understanding, patience, forbearance, gentleness and kindness. He needs to give of his wealth and of himself, and he needs to understand people’s circumstances and habits. Allah says (interpretation of the meaning):
“Invite (mankind, O Muhammad) to the way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Revelation and the Quran) and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His path, and He is the Best Aware of those who are guided.” [al-Nahl 16:125]
Allah blessed His Messenger with the words (interpretation of the meaning):
“And by the Mercy of Allah, you dealt with them gently. And had you been severe and harsh-hearted, they would have broken away from about you; so pass over (their faults), and ask (Allah’s) forgiveness for them; and consult them in the affairs.” [Aal ‘Imran 3:159]
Calling to Islam in the best manner
The da’iyah (caller) may be faced with debates in his da’wah activities, especially with the People of the Book. Allah has commanded us, if it reaches the point of debate, to debate in a manner that is better, which is with kindness and gentleness, explaining the principles of Islam as they came, pure and simple, in a kindly manner and with no element of compulsion. Allah says (interpretation of the meaning):
“And argue not with the people of the Scripture (Jews and Christians), unless it be in (a way) that is better (with good words and in good manner, inviting them to Islamic Monotheism with His Verses), except with such of them as do wrong; and say (to them): ‘We believe in that which has been revealed to us and revealed to you; our Ilah (God) and your Ilah (God) is One (i.e. Allah), and to Him we have submitted (as Muslims).’”
[al-‘Ankabut 29:46]
Reward of da’wah
Calling people to Allah is a virtuous deed which brings great reward. The Prophet (peace and blessings of Allah be upon him) said: “Whoever calls others to guidance will have a reward like the rewards of those who follow him, without that detracting from their reward in any way. And whoever calls others to misguidance will have a burden of sin like the burden of those who follow him, without that detracting from their burden in any way.”(Narrated by Muslim, 2674)
Facing da’wah challenges with patience
If building a physical structure needs effort and patience until it is complete, then building souls and guiding them to the truth also needs patience and sacrifice. The Messenger (peace and blessings of Allah be upon him) called people to Islam, patiently bearing the persecution of the disbelievers, Jews and hypocrites. They made fun of him and disbelieved in him; they insulted him and threw stones at him; they said that he was a sorcerer or a madman; they accused him of being a poet or a soothsayer – but he (peace and blessings of Allah be upon him) bore all that with patience until Allah granted him victory and caused His Religion to prevail. So the da’iyah must follow his example:
“So be patient (O Muhammad). Verily, the Promise of Allah is true; and let not those who have no certainty of Faith discourage you from conveying Allah's Message (which you are obliged to convey) [al-Rum 30:60 – interpretation of the meaning]
The Muslims must follow the example and guidance of their Messenger and call others to Islam , bearing insults and harm with patience for the sake of Allah, as their Messenger (peace and blessings of Allah be upon him) did:
“Indeed in the Messenger of Allah (Muhammad) you have a good example to follow for him who hopes for (the Meeting with) Allah and the Last Day, and remembers Allah much”
[al-Ahzab 33:21 – interpretation of the meaning]
The well-being and happiness of this ummah are only to be found in following this religion. Hence Allah commanded us to convey the message to all of mankind, as He says (interpretation of the meaning):
“This (Quran) is a Message for mankind (and a clear proof against them), in order that they may be warned thereby, and that they may know that He is the only One Ilah (God — Allah) — (none has the right to be worshipped but Allah) — and that men of understanding may take heed.” [Ibrahim 14:52]
And Allah knows best.