He was not that light. So far was John from needing commendation, that the Evangelist gives this warning, lest his excessive brightness might obscure the glory of Christ. For there were some who gazed so eagerly upon him that they neglected Christ; just as if a person, enraptured with beholding the dawning of the day, would not deign to turn his eyes towards the sun. In what sense the Evangelist employs the word light we shall immediately see. All the godly, indeed, are light in the Lord, (Ephesians 5:8,) because, in consequence of their being enlightened by his Spirit, they not only see for themselves, but likewise direct others by their example to the way of salvation. The apostles likewise are peculiarly called light, (Matthew 5:14,) because they go before, holding out the torch of the Gospel, to dispel the darkness of the world. But here the Evangelist speaks of him who is the only and eternal source of illumination, as he immediately shows more clearly.

He was not (o) that Light, but [was sent] to bear witness of that Light.
(o) That light which we spoke of, that is, Christ, who alone can enlighten our darkness.

He was not that light,.... He was a light; he was the forerunner of the sun of righteousness, the "phosphorus" of the Gospel day; he had great light in him; he knew that the Messiah was ready to come, and declared it; and upon his baptism he knew him personally, and signified him to others: he had great light into the person and work of Christ; and into the way of salvation by him, and remission of sins through him; into the doctrines of faith in Christ, and of evangelical repentance towards God; and into the abolition of the legal Mosaic and Jewish dispensation; and was an instrument of giving light to others; yea, he was a burning and shining light, in whose light the Jews rejoiced, at least for a season: but then he was not that light, the word and wisdom of God; that uncreated light that dwelt with him from all eternity; nor that which was the light of men, from the creation; nor that light, which was of old promised to the saints and patriarchs of the Old Testament, and shone in the ordinances and predictions of that state; nor that fountain and giver of light, of every sort, to men; not that light in which is no darkness, and always shines; not that true light, or sun of righteousness, the Messiah, or that lightens every man that comes into the world:
but was sent to bear witness of that light; which is repeated, to distinguish him from that light; to show what he was sent for, and that he acted according to his mission; and to express the honourableness to his work.

not that Light--(See on John 5:35). What a testimony to John to have to explain that "he was not that Light!" Yet was he but a foil to set it off, his night-taper dwindling before the Dayspring from on high (John 3:30).