But that’s not the question, my friend. We are talking about the state of the Faith as held by the Thessalonian Chriistian community around 50 A.D. 2 Thessalonians was one of the first NT items written, certainly before at least three, and probably all four, canonical gospels were written (or known to Paul); and before Acts; and before most of the other Pauline letters; and before all of the other letters; and before Hebrews; and before Revelation – so whatever in the current Catechism is “explicit in Scripture” must necessarily exclude virtually the entire NT. That’s around 80% of the Catechism right there!
Now, it is certainly possible that teachings which later made it into written form in the NT were part of “the traditions which ye have been taught” as referenced by 2 Thess. 2:15. But we don’t know that. What we DO know is that Paul, particularly in the early years of his ministry, was not fully on the same page with the Jerusalem church (including Peter); was not one of the disciples who followed Jesus around; and showed little interest in the details of Jesus’ actual teachings as recorded in the later gospels (because he was concerned with constructing a theology around Jesus’ death, not Jesus’ life).
I’ll stick with the 10% figure. Hey, there is no Time Machine, so we’ll never know!