Really? What was it?
We know it's not the "keys to the kingdom" because those were given to all of the Apostles when they were assembled. (The "keys" were described to Peter as the power to bind and loose on heaven and Earth, Matthew 16:19, but not delivered to Peter alone, but to all of the Apostles, John 20:21-23).
So what other authority was given with Peter's successors mentioned?
and the “rock” is Jesus
“Feed my lambs, Feed my sheep”
Petrine authority has been examined every which way by all theological departments in every major university, by the greatest Catholic theologians, including many who converted to Catholicism, and all over the course 2000 years as attested by the litany of saints and martyrs. This should be enough.
If not go around and see the mudslide of Christian beliefs from Jim Jones and David Koresh to Osteen and Graham, and then of course we have the Mormons, Jehovah’s Witnesses, and Mennonites.