John 1:25 - Albert Barnes' Notes on the Bible

Why baptizest thou 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 the custom. He baptized “all,” and they were desirous of knowing 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 “pretending,” at least, authority from heaven. As he disclaimed the character of Christ and of the prophet Elijah, they asked whence he derived his authority. As he had just before applied to himself a prediction that they all considered as belonging to the fore runner of Christ, they “might” have understood “why” he did it; but they were blind, and manifested, as all sinners do, a remarkable slowness in understanding the plainest truths in religion.