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This verse, placed parenthetically in the midst of the story of Moses and the children of Israel, is meant to elucidate the preceding passage. Each of the earlier prophets was sent to his, and only his, community: thus, the Old Testament addresses itself only to the children of Israel; and even Jesus, whose message had a wider bearing, speaks of himself as "sent only unto the lost sheep of the house of Israel" (Matthew xv, 24). In contrast, the message of the Qur'an is universal - that is, addressed to mankind as a whole - and is neither time-bound nor confined to any particular cultural environment. It is for this reason that Muhammad, through whom this message was revealed, is described in the Qur'an 21:107 as an evidence of "[God's] grace towards all the worlds" (i.e. towards all mankind), and as "the Seal of all Prophets" ( 33:40 ) - in other words, the last of them.
Our attention having been directed to various prophets, who were sent with missions to their several peoples, and in each of whose careers there is some prefigurement of the life of the last and greatest of them, we are now asked to listen to the proclamation of Muhammad's universal mission. We contemplate no longer, after this, partial truths. It is not now a question of saving Israel from the bondage of Egypt, nor teaching Midian the ethics of business, nor reclaiming the people of Lot from sexual sin or Thamud from the sin of oppression in power, or 'Ad from arrogance and ancestorworship. Now are set forth plainly the issues of Life and Death, the Message of Allah, the One Universal God to all mankind.
"Unlettered," as applied to the Prophet here and in verse 157 above, has three special significations. (1) He was not versed in human learning: yet he was full of the highest wisdom, and had a most wonderful knowledge of the previous Scriptures. This was a proof of his inspiration. It was a miracle of the highest kind, a "Sign", which every one could test then, and every one can test now. (2) All organised human knowledge tends to be crystallized, to acquire a partial bias or flavour of some "school" of thought. The highest Teacher had to be free from any such taint, just as a clean slate is necessary if a perfectly clear and bold message has to be written on it. (3) In iii.20 and lxii. 2, the epithet is applied to the Pagan Arabs, because, before the advent of Islam, they were unlearned.