Charles Spurgeon 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)

And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.

It was by baptism that the Christ was to be known. John knew more of Jesus Christ than anybody else, yet he did not know him to be the Lamb of God until he had baptized him.