“All scripture, inspired of God, is profitable to teach, to reprove, to correct, to instruct in justice, that the man of God may be perfect, furnished to every good work” (2 Timothy 3:16-17).
There are so many riches in the Old Testament, and they teach us a great deal about Christ and the New Testament. The idea of original sin is rooted in Genesis, and we see the promise of the Saviour in various places in those books. Why wouldn’t we keep and treasure something that we know is inspired by God? It’s the truth that God gave us - it’s going to be important.
It’s also a wonderful resource to see why Christ is the true Saviour, because of all the prophecies He fulfilled.
“For from the rising of the sun even to the going down, my name is great among the Gentiles, and in every place there is sacrifice, and there is offered to my name a clean oblation: for my name is great among the Gentiles, saith the Lord of hosts.” (Malachias 1:11) - in so many places there are Catholic churches offering the Sacrifice of the Mass.
“For a CHILD IS BORN to us, and a son is given to us, and the government is upon his shoulder: and his name shall be called, Wonderful, Counsellor, God the Mighty, the Father of the world to come, the Prince of Peace. His empire shall be multiplied, and there shall be no end of peace: he shall sit upon the throne of David, and upon his kingdom; to establish it and strengthen it with judgment and with justice, from henceforth and for ever: the zeal of the Lord of hosts will perform this.” (Isaias 9:6-7) - Christ multiplied his empire, the Church, so it embraces such a great number of people. Christ strengthens the same kingdom so the gates of hell will never prevail against it.
Anyway, since the prophets followed God’s revealed truth, they would be Catholics today.