John 1:8

8. οὐκ ἦν ἐκεῖνος τὸ φῶς, ἀλλ' ἵνα μαρτυρήσῃ περὶ τοῦ φωτός.

8. He was not the Light; he was sent to bear witness to the light.

8. Non erat ille lux, sed ut testimonium perhiberet de lumine.

Blue Letter Bible Reference

John Chrysostom (c.346-407)
Homily 6 on the Gospel of John

“He was not that Light.”

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) might secretly harbor in the minds of the simpler sort, so instills into all easily and without impediment the word of doctrine in its proper order.

Let us pray then, that henceforth with the revelation of these thoughts and rightness of doctrine, we may have also a pure life and bright conversation, since these things profit nothing unless good works be present with us. For though we have all faith and all knowledge of the Scriptures, yet if we be naked and destitute of the protection derived from (holy) living, there is nothing to hinder us from being hurried into the fire of hell, and burning for ever in the unquenchable flame. For as they who have done good shall rise to life everlasting, so they who have dared the contrary shall rise to everlasting punishment, which never has an end. Let us then manifest all eagerness not to mar the gain which accrues to us from a right faith by the vileness of our actions, but becoming well-pleasing to Him by these also, boldly to look on Christ. No happiness can be equal to this. And may it come to pass, that we all having obtained what has been mentioned, may do all to the glory of God; to whom, with the Only-Begotten Son and the Holy Ghost, be glory for ever and ever. Amen.

Rev. George Haydock (1774-1849)
Haydock's Catholic Bible Commentary

Ver. 8-9. 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)