Augustus
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Of course our knowledge of Mani's claim to such a title comes through Islamic scholars who were contrasting Mani to their own religion. Carsten Colpe suggests in his paper on the subject that the Manicheans themselves had an entirely different idea on what the term meant, in their case "seal" not being "last" but meaning "mark of authenticity", like a seal on a letter.
So I guess if we include Mani's similar title from a different language the term "Seal of the Prophets" has a fourth potential meaning, though there are no Manicheans anymore, so today there are only three current interpretations of the title.
More than 3 still.
1 Corinthians 9
Am I not free? kAm I not an apostle? lHave I not seen Jesus our Lord? Are not you my workmanship in the Lord? 2 If to others I am not an apostle, at least I am to you, for you are the seal of my apostleship in the Lord.
We turn now to the phrase kha ̄tam al-nabiyy ̄ın in v. 40 of S ̄urat al-Ahz ̆ a ̄b. The first word in this phrase may be read in one of three ways: (1) As a noun, kha ̄tam, which means a seal; (2) as an active participle, kha ̄tim, which means the end or last part or portion; and (3) as a verb, khatama, which means either to seal, stamp, or impress or to reach the end of a thing.62 Let us focus our attention on the first reading (kha ̄tam)—which is the hardest to reconcile with the assertion that Muha ̆ mmad is the Last Prophet. The literal meaning of kha ̄tam al-nabiyy ̄ın is Seal of Prophets. It is possible to understand this figure of speech by analogy to its use in the above-mentioned Jewish, Christian, and Manichean texts. As we have seen, the Qur a ̄n identifies Muha ̆ mmad as a link in a chain of messengers that goes back to Abraham. Previously, God delivered the Torah
to Moses and the Inj ̄ıl to Jesus. In this context, the metaphor in v. 40 may be read as signifying that the revelation sent to Muha ̆ mmad confirms or fulfills the earlier Jewish and Christian revelations. By bearing witness to the truth of the earlier revelations, Muha ̆ mmad placed his personal seal (kha ̄tam) on those texts. This is how some members of the early Muslim community understood the phrase Seal of Prophets.63 (D Powers - Muhammad is not the father of any of your men)