He was not that Light, but was sent to bear witness of that Light.
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Cyprian of Carthage
AD 258
Thus, moreover, John also in his epistle warns us, and says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us; but if we confess our sins, the Lord is faithful and just to forgive us our sins.".
And again, in his epistle, John lays it down, and says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us.".
Also in the Epistle of John: "If we say that we have no sin, we deceive ourselves, and the truth is not in us."
The Baptist having esteemed desert-abodes above the haunts of the cities, and having shewn forth an unwonted persistence in exercise of virtue, and having mounted to the very summit of the righteousness attainable by man, was most rightly wondered at, and even by some imagined to be Christ Himself. And indeed the rulers of the Jews led by his achievements in virtue to some such notion, send some to him bidding them to inquire if he be the Christ. The blessed Evangelist then not ignorant of the things that were by many bruited of him, of necessity puts, He was not the Light, that he might both uproot the error as to this, and again build up some weight of credence to him who was sent from God for a witness. For how is he not eminent exceedingly, how is he not every way worthy of marvel, who is so clad with great virtue and so illustrious in righteousness as to imitate Christ Himself, and by the choice beauty of his piety, to be even imagined to be the Light Itself?
He was not then, s...
He; that is John the Baptist, was not the true light: but the word was the true light. In the translation, it is necessary to express that the word was the true light, lest any one should think that John the Baptist was this light. (Witham)
Now if he did not introduce this as setting himself against this suspicion, then the expression is absolutely superfluous, and tautology rather than elucidation of his teaching. For why, after having said that he was sent to bear witness of that Light, does he again say, He was not that Light? (He says it,) not loosely or without reason; but, because, for the most part, among ourselves, the person witnessing is held to be greater, and generally more trustworthy than the person witnessed of; therefore, that none might suspect this in the case of John, at once from the very beginning he removes this evil suspicion, and having torn it up by the roots, shows who this is that bears witness, and who is He who is witnessed of, and what an interval there is between the witnessed of, and the bearer of witness. And after having done this, and shown His incomparable superiority, he afterwards proceeds fearlessly to the narrative which remains; and after carefully removing whatever strange (ideas)...
And title of brotherhood, and bond.
If we confess our sins, faithful and just is He to remit them to us, and utterly purify us from every unrighteousness."
It often occurs that the man who gives testimony is greater than the one of whom he testifies. Therefore, to prevent the evil thought that John, in bearing witness to Christ, was greater than Christ, the Evangelist says, He was not that Light. But could we not call John, or any one of the saints, a light? Yes, we may call every saint a light. But we cannot call them the Light, with the definite article. If someone says to you, "Surely John is a light?" agree with him. But if he says, "Surely John is the Light," say no. For he is not the very Light Itself, but a light by participation, which derives its brightness from the true Light.