Adam Clarke on John 1:31

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 Joh 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.