Yeshua says that he who practices the Truth comes to the light. What Truth us to be practiced? It is His Torah.
John was apparently told by Elohim to go forth and baptize people with water. When John comes across One who has the Spirit descend upon Him and remaining on Him, he will know that this is the One who will baptize with the Holy Spirit.
The baptism of the Holy Spirit in when the Spirit (or Breath) of Elohim is inside an individual and the heart of that person turns toward Elohim and His Torah,. It is the baptism of the Holy Spirit that cleanses one of sin and makes them a part of the New Covenant (Titus 3:5-6, 1 Cor. 12:13, Acts 11:15-16).