The way and techniques of Da`wah (calling to Islam/Allah) are clarified by Allah (Glorified and Exalted be He) in the Qur'an, and some of them are mentioned in the Sunnah (whatever is reported from the Prophet).
Invite (mankind, O Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭ ﺳﻠﻢ) to the Way of your Lord (i.e. Islâm) with wisdom (i.e. with the Divine Revelation and the Qur'ân) and fair preaching, and argue with them in a way that is better.
Allah (Glorified be He) has clarified the way that should be followed by the Da`ys (callers to Islam/Allah). First, they should begin with wisdom, which refers to the evident, convincing pieces of evidence that reveal the truth and refute falsehood. That is why some interpreters said that it is the Qur'an, as it is the greatest wisdom, and it perfectly reveals the truth. Some others said that it refers to the evidence in the Qur'an and the Sunnah (whatever is reported from the Prophet). Anyhow, wisdom is a great word that means calling to Allah (Exalted be He) on the basis of knowledge, insight and solid, convincing evidence. It is a common word that is given for many concepts, including prophethood, knowledge, understanding religion, reason, and piety. It is similar to what AlShawkany (may Allah be merciful with him) said, "Wisdom is the thing that protects from foolishness." This means that every word that protects you from foolishness, as well as anything that keeps you away from falsehood; and every clear, true statement is considered wisdom. The Qur'an is the most worthy of the designation 'wisdom', and next comes the authentic Sunnah.
and instruct them in the Book (this Qur'ân) and AlHikmah (full knowledge of the Islâmic laws and jurisprudence or wisdom or Prophethood)
He grants Hikmah to whom He pleases, and he, to whom Hikmah is granted, is indeed granted abundant good.
Wisdom is a word that prevents someone who hears it from being misled by falsehood, and calls them to adhere to the truth and abide by the limits of Allah (Glorified and Exalted be He).
A Da`y should call people with wisdom. If the invited person is reluctant, you should use fair preaching, Ayahs and Hadiths that encourage him. If he has any doubts, you should argue with him with the better preaching, be patient, use the Qur'an and the Sunnah that include temptation and intimidation. You should do your best to clarify any doubtful matter with clear evidence in a reasonable manner. You should be patient, and never be cruel. This is the best way for the invited person to accept the truth and you should always be patient in argument. When Allah (Glorified and Exalted be He) sent Harun (Aaron) and Musa (Moses) (peace be upon them) to Pharaoh, He asked them to speak to him gently, even though he was the cruelest tyrant.
And speak to him mildly, perhaps he may accept admonition or fear (Allâh).
And by the Mercy of Allâh, you dealt with them gently. And had you been severe and harshhearted, they would have broken away from about you
Thus, it is known that the wise, straight way of Da`wah is being wise, knowledgeable of its ways, and never being cruel or hasty, but rather addressing people with wisdom, which refers to the clear, truthful sayings of the Qur'an and Hadith; fair preaching and gentle argument. This is the approach that should be adopted while practicing Da`wah. On the other hand, Da`wah based on ignorance is harmful, and never be useful, as we will mention later when describing the morals of Da`ys. Practicing Da`wah while being ignorant of evidence, is just like speaking about Allah (Exalted be He) without knowledge. Approaching Da`wah by the use of violence brings more harm than benefit.
Invite (mankind, O Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭ ﺳﻠﻢ) to the Way of your Lord (i.e. Islâm) with wisdom (i.e. with the Divine Revelation and the Qur'ân)
O Prophet (Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭﺳﻠﻢ)! Strive hard against the disbelievers and the hypocrites, and be harsh against them
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 Islâmic Monotheism with His Verses), except with such of them as do wrong