It wasn't Peter. The title goes back to at least 600 BC.
Now, why was that title, that of the Roman emperor, retained? Caesar, Nero and Caligula held that title well before. Not very pleasant company.
BFLR
did Caesar, Nero and Caligula believe that Jesus Christ is true God and true man, who lived a sinless life and willingly sacrificed His life to reconcile sinners to the Father? is that the “mystery religion” you are talking about?
here i thought these three clowns thought they were gods, and not the Trinity.
i guess i was sleeping in history class when this was discussed.