Blessings on those who spread the knowledge: Prophet (blessings and peace of Allah be upon him) said:
“Allah, the angels, the inhabitants of heaven and earth, even the ant in its hole and even the fish in the sea, send blessings upon the one who teaches the people good.”
Narrated by al-Tirmidhi, 2609; classes as saheeh by al-Albaani in Saheeh al-Jaami‘, 1838.
By the grace of Allah, the one who teaches the people something about their religion deserves this supplication, because what he taught them will bring them to something good.
In Saheeh al-Bukhaari (3461) it is narrated from ‘Abd-Allah ibn ‘Amr (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said:
Convey from me, even if it is a verse
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Al-Haafiz Ibn Hajar said in al-Fath: And he said in the hadeeth: “even if it is a verse,” meaning just one, so that everyone who heard it would hasten to convey whatever verses he knew, even if it was a little, so that by this means everything he (the Prophet (blessings and peace of Allah be upon him)) brought would be transmitted. End quote.
It is not essential to know the entire religion to call people to Islam: But the one who takes on the mission of calling people to Islam has to have knowledge and insight into what he is calling them to. It is not essential for him to know the entire religion. Explaining this issue, Ibn ‘Uthaymeen said:
If a person has proper understanding what he is calling people to, it makes no difference whether he is a great scholar or a seeker of knowledge who is a hard worker, or an ordinary Muslim whose knowledge of a particular issue is clear and certain. The Messenger (blessings and peace of Allah be upon him) said: “Convey from me, even if it is a verse.” He did not stipulate that the daa‘iyah (one who calls people to Allah) should have vast knowledge; rather he stipulated that he should have knowledge of what he is calling people to. As for the one who is ignorant and calls people on the basis of emotion, that is not permissible.
Hence we see some brothers who call people to Allah although they have only a little knowledge, and we find that because of the strength of their emotions they declare to be haraam things that Allah has not declared haraam, and they impose obligations that Allah has not enjoined upon His slaves. This is a very serious matter. Allah says (interpretation of the meaning):
“And say not concerning that which your tongues put forth falsely: ‘This is lawful and this is forbidden,’ so as to invent lies against Allaah. Verily, those who invent lies against Allaah will never prosper.
117. A passing brief enjoyment (will be theirs), but they will have a painful torment”
[al-Nahl 16:116, 117].
http://www.islamqa.com/en/ref/103895