Matthew Poole and continuators on John 1:8

8 He was not that light , but was sent to bear witness of that light .

Iohn the Baptist was a Light, as all Saints are light in the Lord, Eph. 5.8. Nay, in a peculiar sense our Saviour beareth him Witness, that he was a burning and shining light; but he was not that light before mentioned, v. 5. that shineth in darkness; and again v. 9. which lighteth every one that cometh into the World. Iohn borrowed his Light from that Original Light; that Light was God, he was but a man sent from God. The men of the World are ordinarily in extreams, either wholly rejecting Gods Ministers and Witnesses, or else adoring them; as the World is concerned to take heed of the former, so the Ministers of Christ are also highly concerned not to admit the latter. See Luke 7.33. Acts 14.13, 14. but both Iohn here, and Paul there, were very cautious not to rob their Master of the honour due unto him alone.

Iohn, as was said before, came onely to bear Witness of that Light, that he was come, and shined forth, and was the true Light, as it followeth.