Albert Barnes Commentary John 1:33-34

"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. And I have seen, and have borne witness that this is the Son of God." — John 1:33-34 (ASV)

The same said ... - This was the sign by which he was to know the Messiah. He was to see the Spirit descending like a dove and abiding on him. It does not follow, however, that he had no intimation before this that Jesus was the Christ, but it means that by this he should know it infallibly. From Matthew 3:13-14, it seems that John supposed, before the baptism of Jesus, that he claimed to be the Messiah, and that he believed it; but the infallible, certain testimony in the case was the descent of the Holy Spirit on him at his baptism.

That this is the Son of God - This was distinctly declared by a voice from heaven at his baptism, Matthew 3:17. This John heard, and he testified that he had heard it.