And I myself did not know him, but the one who sent me to baptize with water told me, ‘The man on whom you see the Spirit come down and remain is the one who will baptize with the Holy Spirit.’
John repeats his ignorance of the fact that his cousin was the Messiah. But he lets on that he has received a message from God that will allow him to identify the Messiah. Jesus will baptize with the Spirit. Water baptism was the sign, Spirit baptism was the thing signified.
Although Jesus' ministry is not particularly associated with baptism, John ch 3 tells that Jesus did baptize early in his ministry. But the way that Jesus conferred the Spirit was by his death and resurrection. In Romans ch 6 Paul tells us that we are baptized into his death and are buried with him through baptism in order that just as Christ was raised from the dead we too may live a new life. The new life can only be lived in the Spirit. In the book of Acts, when a believer was baptized in water he was also baptized by the Spirit and received in his flesh the gifts of the Spirit to work out his own salvation and display in his life the fruits of the Spirit. Nowadays, baptism, whether of infants or believers, is seldom accompanied by those spectacular gifts - speaking in tongues, miraculous healings; why is this?
Baptism is not essential for salvation; the thief on the cross was not baptised and many in the Quakers and Salvation Army are saved without baptism. Yet it is the Lord's command, "Repent and be baptized, every one of you."
However, baptism is the sign not the substance. The Spirit is conferred when we believe.