The Popes change. The Word of God, (Bible) does not change.
You seem to hav missed the point. No dimunition of Scripture, just a fact-check to ensure that people understand that having one common pastor as an earthly leader is as Christian as you can get. That is what united the Church for 1500 centuries. And certainly for the first 300 years when there was no NT.
The Catholic Church eventually declared that the books of the NT were all the Word of God. Prior to that they were just books that had good teaching in them. Contemporary books of similar stature up until that point (e.g. other "gospels") were NOT declared to be the Word of God by the Catholic Church and thus do not have that status today.
Popes are bound by Catholic doctrine, so the pope can't mess doctrine up. The pope may be infallible in theory, but the exercise of that infallibility is almost non-existent, and has only been exercised regarding things long accepted as true - since the days of the orignial apostles. The Word of God does change - all yo have to do is go to a bibkle store and ten compare the text of the different versions.