AM: If you are correct, the NT Church was founded on something that didn't exist, because not one word of the NT Scriptures was written for years after the Church came into existence. You have it all exactly backwards. It was the Church that produced the Scriptures, used them liturgically, and distinguished between those to be considered inspired and those not.
He is correct. Scripture certainly did exist before the RCC came along and claimed to have written it.
Jesus referred to it plenty of times in the Gospels. Paul referred to Scripture in his letters, when talking to Timothy. The Bereans were commended for searching the Scripture daily to see if those things Paul was teaching were so.
And what scripture would that be? What we call the OT.
The Catholic church simply cannot take credit for that. Nor is anyone else accepting their taking credit for the rest of the NT which was written primarily by Jewish people. Claiming they are Catholic retroactively is simply a bid to claim authority over Scripture and it's obvious what's going on there.
Peter believed the writings of Paul to be scripture as he included them with scripture.
2 Peter 3:15b. As also our beloved brother Paul wrote to you, according to the wisdom given him, [16] as in all his letters, speaking concerning these matters, in which some things are hard to understand, which those who are untaught and unstable twist to their own destruction, as they do also THE OTHER SCRIPTURES. [17] You then, beloved ones, being forewarned, watch lest you also fall from your own steadfastness, being let away with the delusion of the lawless.
And as said, if recognizing writings as being of God makes one the supreme authority, in the form of the magisterium, then the church should have submitted to the scribes and the Pharisees.