And I knew him not. That his testimony may not be suspected of having been given either from friendship or favor, he anticipates such a doubt, by affirming that he had no other knowledge of Christ than what he had obtained by divine inspiration. The meaning, therefore, amounts to this, that John does not speak at his own suggestion, nor for the favor of man, but by the inspiration of the Spirit and the command of God. I came baptizing with water; that is, I was called and appointed to this office, that I might manifest him to Israel; which the Evangelist afterwards explains more fully, and confirms, when he introduces John the Baptist, testifying that he had no knowledge of Christ but what he had obtained by oracle; that is, by information or revelation from God. [1] Instead of what we find here, I came to baptize, he there states expressly (verse 33) that he was sent; for it is only the calling of God that makes lawful ministers, because every person who of his own accord, thrusts himself forward, whatever learning or eloquence he may possess, is not entitled to any authority, and the reason is, that he is not authorized by God. Now since it was necessary that John, in order that he might lawfully baptize, should be sent by God, let it be inferred from this, that it is not in the power of any man whatever to institute sacraments, but that this right belongs to God alone, as Christ, on another occasion, in order to prove the baptism of John, asks if it was from heaven, or from men, (Matthew 21:25.)

Footnotes

1 - "Par oracle; c'est a dire, advertissement ou revelation de Dieu."

I knew him not - John was not personally acquainted with Jesus. Though they were remotely related to each other, yet it seems that they had had heretofore no personal acquaintance. John had lived chiefly in the hill country of Judea. Jesus had been employed with Joseph at Nazareth. Until Jesus came to be baptized Matthew 3:13-14, it seems that John had no acquaintance with him. He understood that he was to announce that the Messiah was about to appear. He was sent to proclaim his coming, but he did not personally know Jesus, or that he was to be the Messiah. This proves that there could have been no collusion or agreement between them to impose on the people.
Should be made manifest - That the Messiah should be "exhibited," or made known. He came to prepare the way for the Messiah, and it now appeared that the Messiah was Jesus of Nazareth.
To Israel - To the Jews.

And I knew him not, etc. - John did not know our Lord personally, and perhaps had never seen him, at the time he spoke the words in John 1:15. Nor is it any wonder that the Baptist should have been unacquainted with Christ, as he had spent thirty years in the hill country of Hebron, and our Lord remained in a state of great privacy in the obscure city of Nazareth, in the extreme borders of Galilee.
But that he should be made manifest to Israel - One design of my publicly baptizing was, that he, coming to my baptism, should be shown to be what he is, by some extraordinary sign from heaven.

And (q) I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
(q) I never knew him by face before.

And I knew him not,.... "by sight", as Nonnus paraphrases it; personally he had never seen him, nor had had any conversation and familiarity with him; for though they were related to each other, yet lived at such a distance, as not to know one another, or have a correspondence with each other: John was in the deserts, until the day of his showing unto Israel; and Christ dwelt with his parents at Nazareth, in a very mean and obscure manner, till he came from thence to Jordan to John, to be baptized by him; and which was the first interview they had: and this was so ordered by providence, as also this is said by John, lest it should be thought, that the testimony he bore to Jesus, and the high commendation he gave of him, arose from the relation between them; or from a confederacy and compact they had entered into:
but that he should be made manifest to Israel; who had been for many years hid in Galilee, an obscure part of the world: and though he had been known to Joseph and Mary, and to Zacharias and Elisabeth, and to Simeon and Anna; yet he was not made manifest to the people of Israel in common; nor did they know that the Messiah was come: but that he might be known:
therefore am I come baptizing with water; or in water, as before: for by administering this new ordinance, the people were naturally put upon inquiry after the Messiah, whether come, and where he was, since such a new rite was introduced; and besides, John, when he baptized any, he exhorted them to believe on him, which should come after him, that is, on Christ Jesus; and moreover, by Christ's coming to his baptism, he came to have a personal knowledge of him himself, and so was capable of pointing him out, and making him manifest to others, as he did.

knew him not--Living mostly apart, the one at Nazareth, the other in the Judean desert--to prevent all appearance of collusion, John only knew that at a definite time after his own call, his Master would show Himself. As He drew near for baptism one day, the last of all the crowd, the spirit of the Baptist heaving under a divine presentiment that the moment had at length arrived, and an air of unwonted serenity and dignity, not without traits, probably, of the family features, appearing in this Stranger, the Spirit said to him as to Samuel of his youthful type, "Arise, anoint Him, for this is He!" (1-Samuel 16:12). But the sign which he was told to expect was the visible descent of the Spirit upon Him as He emerged out of the baptismal water. Then, catching up the voice from heaven, "he saw and bare record that this is the Son of God."

I knew him not. Knew not that God had chosen him to be the Christ. He knew, however, that he should be manifested in some way through his baptism.

I knew him not - Till he came to be baptized. How surprising is this; considering how nearly they were related, and how remarkable the conception and birth of both had been. But there was a peculiar providence visible in our Saviour's living, from his infancy to his baptism, at Nazareth: John all the time living the life of a hermit in the deserts of Judea, Luke 1:80, ninety or more miles from Nazareth: hereby that acquaintance was prevented which might have made John's testimony of Christ suspected.