A ŭ One who invites to Islam has to know what he is ordering and what he is inviting to. He has to be gentle and patient. But he doesnŭt have to be one who has memorized the Qurŭan in its entirety, nor does he have to be thoroughly acquainted with the fine details of any branch of knowledge. What is required is that he must have learnt the obligations and prohibitions of the religion that he is inviting others to. Thereafter, he should learn the proofs that are convincing enough to those he is inviting. It is also befitting to learn the methods of oration, and how to make the listener understand and appreciate what he is calling to.
ŭ The Permanent Committee for Islamic Research and Verdicts; Fatawa Islamiyah, vol. 8 no. 46.
http://saudigazette.com.sa/index.cfm?method=home.regcon&contentID=2009042736277
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Q 1: What are the means whereby a Da`wah becomes successful and how should they be deduced? What are the essential characteristics of Da`y (caller to Islam)? We hope that you will mention some of the important books in this field.
See the full answer here
There are currently a number of other da'wah Q & A on the al-Ifta Homepage: http://www.alifta.com/Default.aspx
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... Third, some of the prerequisites that should be available in the Da`y are mentioned in the story of Shu'ayb, for Allah says: هود, 11: 88 He said: {"O my people! Tell me if I have a clear evidence from my Lord and He has given me a good sustenance from Himself (shall I corrupt it by mixing it with the unlawfully earned money). I wish not, in contradiction to you, to do that which I forbid you. I only desire reform to the best of my power. And my guidance cannot come except from Allah, in Him I trust and to Him I repent.} This Ayah (Qur'anic verse) shows that the conditions of successful Daw`ah that includes: knowledge, good earning, obeying what Allah prescribes and avoiding what He forbids, good intention, leaving all matters to Allah (may He be Exalted), and putting one's trust in Allah, as He is the One; the Giver of success and inspiration.
Also pertaining to the prerequisites that Allah (may He be Exalted) mentions in His saying: النحل, 16: 125 {Invite (mankind, O Muhammad peace be upon him) to the Way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Revelation and the Qur'an), and fair preaching, and argue with them in a way that is better.} He should also be patient, as Allah says: النحل, 16: 127 {And endure you patiently (O Muhammad peace be upon him), your patience is not but from Allah.} and Allah says: الكهف, 18: 28 {And keep yourself (O Muhammad peace be upon him) patiently with those who call on their Lord (i.e. your companions who remember their Lord with glorification, praising in prayers, and other righteous deeds) morning and afternoon, seeking His Face; and let not your eyes overlook them, desiring the pomp and glitter of the life of the world; and obey not him whose heart We have made heedless of Our Remembrance, and who follows his own lusts, and whose affair (deeds) has been lost.}
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BaarakAllaahu feekum.