John Gill

This is he, of whom it is said,.... Either the day before, as in Joh 1:27, or some time before that, Joh 1:15, when he first began to baptize, even before Christ came to be baptized by him, and before he personally knew him; see Mat 3:11. After me cometh a man; not a mere man, but the man God's fellow: and this is said, not because he was now a grown man, or to show the truth of his human nature; but seems to be a common Hebraism, and is all one as if it had been said, "after me cometh one", or a certain person: for the sense of this phrase, and what follows; see Gill on Joh 1:15.