"Ultimately, there is one set of keys and it is in the hand of the successor of St. Peter."
Actually there are more than one set of keys;
Rev 1:18 "I [am] he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death."
Rev 3:7 "And to the angel of the church in Philadelphia write; These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth;"
Rev 9:1 "And the fifth angel sounded, and I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit."