John 1:25

Why baptizest thou then? - Baptism was a very common ceremony among the Jews, who never received a proselyte into the full enjoyment of a Jew's privileges, till he was both baptized and circumcised. But such baptisms were never performed except by an ordinance of the Sanhedrin, or in the presence of three magistrates: besides, they never baptized any Jew or Jewess, nor even those who were the children of their proselytes; for, as all these were considered as born in the covenant, they had no need of baptism, which was used only as an introductory rite. Now, as John had, in this respect, altered the common custom so very essentially, admitting to his baptism the Jews in general, the Sanhedrin took it for granted that no man had authority to make such changes, unless especially commissioned from on high; and that only the prophet, or Elijah, or the Messiah himself; could have authority to act as John did. See the observations at the conclusion of Mark.

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.

1:25 They asked him, Why baptizest thou then? - Without any commission from the sanhedrim? And not only heathens (who were always baptized before they were admitted to circumcision) but Jews also?