Adam Clarke on John 1:15

Of him - The glorious personage before mentioned: John the Baptist, whose history was well known to the persons to whom this Gospel came in the beginning, bare witness; and he cried, - being deeply convinced of the importance and truth of the subject, he delivered his testimony with the utmost zeal and earnestness, - saying, This is he of whom I spake, He that cometh after me - for I am no other than the voice of the crier in the wilderness, Isa 40:3, the forerunner of the Messiah.

Was before me - Speaking by the prophets, and warning your fathers to repent and return to God, as I now warn you; for he was before me - he was from eternity, and from him I have derived both my being and my ministry.