John 1:33

And I knew him not
That is, before he came to be baptized by him; when it was secretly suggested to him who he was, and the following signal was given him, to confirm him in it:

but he that sent me to baptize with water;
or "in water"; that is, God; for John's mission was from God, as in ( John 1:6 ) , and his baptism from heaven; he had a divine warrant and commission for what he did:

the same said unto me;
either by an articulate voice, or by a divine impulse on his mind, or by the revelation of the Spirit:

upon whom thou shalt see the Spirit descending and remaining on him,
the same is he which baptizeth with the Holy Ghost;
that is, the Messiah; (See Gill on Matthew 3:11), (See Gill on Matthew 3:16).