Well a lot of it was written down - many documents, Church documents, Papal documents, etc. exist that are not inspired Scripture.
A lot of it is also passed down in oral tradition - protected by the Holy Spirit, and confirmed in the Bible as one of the ways Jesus wanted the Church to continue His mission.
If you have concerns about this, research when the various parts of the New Testament were actually written, and it’s longer than from now back to the Civil War

, so why would one believe that? Why do we believe the writings of Matthew, Mark, Luke, and John are inspired, since the Bible doesn’t directly say so? (If you don’t believe that read the Bible some time and imagine you never heard the concept of “Inspired” - would you understand the Bible was afterward? I doubt it, we get that from tradition)