And they asked him, and said unto him, {l} Why baptizest thou then, if thou be not that Christ, nor Elias, neither that prophet?

(l) By this we may prove that the Jews knew there should be some change in religion under the Messiah.

People's New Testament

1:25 Why baptizest thou then? If he were Christ, or Elijah, or that prophet, they could understand why he should establish a new religious rite, but if none of these, why should he do so? Their perplexity shows that the baptismal rite was new to them. There is no proof that Jewish proselyte baptism of Gentile converts existed at this period, save the assertion of the Talmud, written two or three centuries after this. Josephus, who wrote in the time of the apostles, is silent about it.

Wesley's Notes

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?

Jamieson-Fausset-Brown Bible Commentary

25. 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.

Matthew Henry's Concise Commentary

1:19-28 John disowns himself to be the Christ, who was now expected and waited for. He came in the spirit and power of Elias, but he was not the person of Elias. John was not that Prophet whom Moses said the Lord would raise up to them of their brethren, like unto him. He was not such a prophet as they expected, who would rescue them from the Romans. He gave such an account of himself, as might excite and awaken them to hearken to him. He baptized the people with water as a profession of repentance, and as an outward sign of the spiritual blessings to be conferred on them by the Messiah, who was in the midst of them, though they knew him not, and to whom he was unworthy to render the meanest service.