John 1:25

“And they asked him, and said unto him, etc.] They put a question, by saying to him, why baptizest thou then, if thou be not that Christ, nor Elias, neither that prophet? since he denied that he was the Messiah, or Elias that was to come before the Messiah, according to the expectation of the Jews, or that prophet, or a prophet, they demand by what authority he introduced a new rite and ordinance among them, which they had never been used to; for though there were divers washings or baptisms among them, enjoined by the law of Moses in certain cases, and others which obtained by tradition, as the immersion of themselves after they had been at market, and of cups, pots, brazen vessels, and…”

“Why baptizest thou, if not, &c.--Thinking he disclaimed any special connection with Messiah's kingdom, they demand his right to gather disciples by baptism.”

“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 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? John 1:26”

“1:25-26 Jewish baptisms were ritual washings for becoming ceremonially pure following contact with impurity (Lev 8:6; Num 19:7). • John’s announcement of the Messiah’s arrival required that participants confess their sins and be baptized (Matt 3:6). Baptism later became the symbol of membership in Jesus’ kingdom (Acts 2:38; see also John 4:1-2).”