Albert Barnes 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 do you baptize then... — Baptism on receiving a proselyte from paganism was common before the time of John, but it was not customary to baptize a Jew. John had changed this custom. He baptized everyone, and they desired to know by what authority he made such a change in the religious customs of the nation.

They presumed, from the fact that he introduced that change, that he claimed to be a prophet or the Christ. They supposed that no one would attempt it without at least pretending to have authority from heaven. Since he disclaimed being the Christ or the prophet Elijah, they asked from where he derived his authority.

Since he had just before applied to himself a prediction that they all considered as belonging to the forerunner of Christ, they might have understood why he did it. However, they were blind and displayed, as all sinners do, a remarkable slowness in understanding the plainest truths in religion.