John Gill Commentary John 1:33

"And I knew him not: but he that sent me to baptize in water, he said unto me, Upon whomsoever thou shalt see the Spirit descending, and abiding upon him, the same is he that baptizeth in the Holy Spirit." — John 1:33 (ASV)

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 you shall see the Spirit descending and remaining on him ,
the same is he which baptizes with the Holy Ghost ;
that is, the Messiah; (See Gill on Matthew 3:11), (See Gill on Matthew 3:16).