Ali (A.S.), the First amongst Men to Adopt Islam

Mamun's Discussion with Ishaq
Ibn Abd Rabbih quotes an interesting incident in Aqdul Farid' which may be summarized as under:
Mamun arranged a debating assembly and the famousscholar ishaq occupied the foremost position in it. When Ali's precedence over others in the matter of acceptance of Islam was established, Ishaq said: "When Ali embraced Islam he was only a boy but Abu Bakr was a mature man (at the time he became a Muslim). Hence his faith enjoys superiority over that of Ali .
Mamun suddenly intervened and said: "Did the Prophet invite Ali in his boyhood to adopt the faith, or his faith was due to divine inspiration? It cannot at all be said that his faith was inspired, because, not to speak of Ali, even the faith of the Prophet was not inspired but was the result of guidance and messages brought by Jibreel from Allah. Hence, when the Holy Prophet invited him to accept Islam, did he do so on his own account or had he been ordered by Allah to accomplish the deed? We cannot imagine that the Prophet should subject himself or anyone else to hardship and responsibility without an order from Allah. Hence there is no alternative except that we should say that the Prophet's call is supported by divine order. And does the Omniscient Lord order His Prophet to invite an untalented boy (for whom faith' or no faith' are alike) to adopt Islam? Certainly such an act is not possible from the All-Wise and All-Knowing Allah.
Hence, it should be concluded that the faith of Ali was a true and firm faith which was not at all inferior to the faith of others and it is Ali son of Abu Talib to whom the verses of Qur'an "and the foremost are the foremost, these are they who are drawn nigh (to Allah)" , most appropriately applies.