A.T. Robertson Commentary John 1:25

"And they asked him, and said unto him, Why then baptizest thou, if thou art not the Christ, neither Elijah, neither the prophet?" — John 1:25 (ASV)

Why then baptizest thou? (Τ ουν βαπτιζεισ; ). In view of his repeated denials (three here mentioned).

If thou art not (ε συ ουκ ε). Condition of first class. They did not interpret his claim to be "the voice" to be important enough to justify the ordinance of baptism. Abrahams (Studies in Pharisaism and the Gospels) shows that proselyte baptism was probably practised before John's time, but its use by John was treating the Jews as if they were themselves Gentiles.