John 1:25 - Ellicott's Commentary On The Whole Bible

Why baptizest thou then? — Baptism, which was certainly one of the initiatory rites of proselytes in the second or third century A.D., was probably so before the work of the Baptist. It is not baptism, therefore, which is strange to the questioners, but the fact that he places Jews and even Pharisees (Matthew 3:7) in an analogous position to that of proselytes, and makes them to pass through a rite which marks them out as impure, and needing to be cleansed before they enter “the kingdom of heaven.” By what authority does he these things? They had interpreted such passages as Ezekiel 36:25 ff. to mean that Baptism should be one of the marks of Messiah’s work. None less than the Christ, or Elias, or “the prophet” could enact a rite like this. John is assuming their power, and yet is not one of them.