John Calvin Commentary John 1:31

"And I knew him not; but that he should be made manifest to Israel, for this cause came I baptizing in water." — John 1:31 (ASV)

And I knew him not. So that his testimony might not be suspected of having been given either from friendship or favor, he anticipates such a doubt by affirming that he had no other knowledge of Christ than what he had obtained by divine inspiration. The meaning, therefore, amounts to this: John does not speak on his own initiative, nor for human favor, but by the inspiration of the Spirit and the command of God.

I came baptizing with water; that is, I was called and appointed to this office, that I might manifest him to Israel. The Evangelist later explains and confirms this more fully when he introduces John the Baptist, testifying that he had no knowledge of Christ other than what he had obtained by oracle—that is, by information or revelation from God.

Instead of what we find here, "I came to baptize," he states there expressly (John 1:33) that he was sent. For it is only the calling of God that makes lawful ministers, because every person who, on his own initiative, thrusts himself forward, whatever learning or eloquence he may possess, is not entitled to any authority, because he is not authorized by God.

Now, since it was necessary for John to be sent by God in order to baptize lawfully, it may be inferred from this that no human being has the power to institute sacraments, but that this right belongs to God alone—as Christ, on another occasion, in order to prove John’s baptism, asks if it was from heaven, or from men (Matthew 21:25).