John 1:30

This is he, of whom it is said,.... Either the day before, as in John 1:27, or some time before that, John 1:15, when he first began to baptize, even before Christ came to be baptized by him, and before he personally knew him; see Matthew 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 "John 1:15".