Peter 1:21 (KJV) For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
Verse 20 is not saying the Catholic church is God's official interpreter, it is saying that man didn't think up a prophecy: God did via the Holy Ghost.
From the Catholic Bible ... here is the footnote from that particular verse.
Often cited, along with 2 Tim 3:16, on the "inspiration" of scripture or against private interpretation, these verses in context are directed against the false teachers of 2 Peter 2 and clever tales (2 Peter 1:16). The prophetic word in scripture comes admittedly through human beings (2 Peter 1:21), but moved by the holy Spirit, not from their own interpretation, and is a matter of what the author and Spirit intended, not the personal interpretation of false teachers. Instead of under the influence of God, some manuscripts read "holy ones of God."
Bingo, the Holy Spirit sent to the Church as a whole on Pentacost. It is the Holy Spirit who has guided the Church in all truth and the same Holy Spirit guides the Church today. Alleluia! Thanks be to God for His wonderful gift.