I think if I were to summarize this section in one phrase it would be, “remember always the unity of Scripture”. While some may think the canon (i.e. the defined list of books) of the Catholic Bible was first set at the Council of Trent it in fact traces back all the way to the Council of Rome in 382. We find the canon delineated for us in Paragraph 120. That kind of misunderstanding seems to be a recurring theme – that Councils or Popes define something new for belief that was never thought of before. The reality, however, is precisely the opposite – Councils and Popes codify that which is already believed so as to prevent confusion and confute error.
After this we are reminded not once but twice (in Paragraphs 121 and 123) of the importance of the Old Testament. To me, at least, it is still amazing that after all this time the Marcionist heresy still makes its rounds, but Paragraph 123 states the continued validity and importance of the Old Testament forcefully, “The Church has always vigorously opposed the idea of rejecting the Old Testament under the pretext that the New has rendered it void.” We’ll hear about this in another distorted way from the New Atheists who will cheerily chime in with erudite observations as, “you shouldn’t be eating pork or shellfish either”. That fundamentally is not how the Church understands the continued validity of the Old Testament.
Indeed I would say that the New Testament is virtually impossible to understand to any great depth without some attention paid to the Old and, truthfully, the Old Testament only begins to truly make sense when read in concert with the New. “Christians therefore read the Old Testament in the light of Christ crucified and risen.” (Paragraph 129) We do not read lines or paragraphs or chapters or even whole books in isolation but rather take the entirety as the organic whole that it is. Only in this way can you properly understand what God is trying to tell you through His Word given to us down through history. As Augustine once heard, “tolle lege” – “take up and read”.