Albert Barnes Commentary John 1:31

"And I knew him not; but that he should be made manifest to Israel, for this cause came I baptizing in water." — John 1:31 (ASV)

I knew him not - John was not personally acquainted with Jesus. Though they were remotely related to each other, it seems that they had previously had 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.