John 1:25 - Coke's Commentary on the Holy Bible

Why baptizest thou then,— The Jews, it seems, had conceived an opinion that they were all to be baptized, either by Messiah himself, or by some of his retinue; which they drew from Zechariah 13:1.though that prophecy is to be taken in a most spiritual sense. The decisions of the Pharisees were held by the common people as infallible; wherefore they are mentioned John 1:24.; and as this sect had determined that onlyproselytes were to be baptized, they found fault with John for baptizing the Jews, seeing he was neither the Messiah, nor Elias,nor a prophet. They thought his altering, in this manner, their institutions, was an exercise of authority, which, by his own confession, did not belong to him. It is not to be certainly determined from this text, whether the baptism of proselytes was then in use among the Jews or not. The words indeed will make a strong and well-adapted sense, should they be understood as if it had been said, "Why is it then that thou dost institute such a new rite as this?" But surely too they will be very proper in the other sense, if we understand them to imply, "Why is it then that thou takest upon thee, without any commission from the Sanhedrim, to administer baptism; and that not only, as is usual, unto those who before this were heathens, but even to the Jews?" And this seems to be the more probable sense; while the Baptist's use of this ceremony in such a manner was a strong intimation that Jews, as well as Gentiles, must become proselytes to the new dispensation which was then opening to the world; and that however holy they imagined themselves, yet they all stood in absolute need of being washed from their sins, as he had before most emphatically declared, Matthew 3:8-9.