Right, it's a belief, an assumption if you will. But as far as the Bible is concerned, the only words attributed to God are a priori those that are quoted as such, i.e. that "come out of the mouth of God".
Paul, for his part, did not specify what (of which) books constitute scripture. On several occasions, he states that what he is saying are his own commandments and "not the commandments of the Lord".
Even he realized the difference.
I will just skip on down. The net result must be that the Roman Catholic Church:
1. claims authority based upon the Bible,
2. of which parts may not even be the Word of God, but
3. which it alone is capable of interpreting.
Got it.