Most Protestants don’t think back to the time when there was no New Testament. What guided the Church until the NT Canon was identified? That would be Tradition which included OT Scripture - including the Septuagint. What part does Tradition play now? Most don’t know that sola scriptura doesn’t mean only Scripture. It means that the Bible is the only infallible rule of faith and practice. All other authorities are fallible. And that’s a problem. In the Bible the Holy Spirit is said to have breathed out Scripture, making it infallible and absolutely authoritative. In the Bible the Holy Spirit birthed the Church and filled her with infallible discernment, leading her into all truth. The common thread? The work of the Holy Spirit who cannot lie or fail. It’s absurd to say that Scripture is infallible and God-breathed, but the Church is fallible and unable to discern what truth is. The fallible-infallible model of sola scriptura is self refuting.
Even more absurd is the fact that most Protestants do accept the fact that the Church is led into all truth and somehow got the NT Canon right. Even the Lutheran church saw the madness that Luther was leading it into with his ideas on what books belonged and what books did not belong in the NT! The guy was on a tear.
Hebrews, James, Jude, Revelation - and maybe others - didn’t meet his criteria, but the Lutheran church had the good sense to veto him on that.
Most don’t know the history of the OT canon - or canons, rather - and how they came to be. Most don’t know that the Protestant OT Canon is not older than the 1st century AD.
Protestant tradition about the OT canon was determined by Martin Luther and then Bible publishers in the 19th Century. I’ve written about that in other posts. Luther decided that the Hebrew OT was the correct canon. There are problems with that, like the fact that NT writers quoted and referenced the Greek OT.
Here’s a list of Deuterocanonical references in the NT. This is a big deal. If the books that Luther removed from their place in the OT and put in the back of his Bible translation are really part of the OT canon, then there’s a big problem for Protestants’ view of sola scriptura.
No matter. Read your Bible - the one sitting on your shelf that has been unopened. All Christians have a common NT canon, so don’t worry about the other stuff. Take and read - which is what St. Augustine did.
Ask the Holy Spirit to enlighten you. But be careful what teachers you choose to help you.
Stick with the tried and true. Get familiar with the Church fathers especially St. Augustine and St. Chrysostom. They are clear and it’s as if they were speaking to us today. Yes, the Holy Spirit will enlighten you, but not all on your own.
Go to the local perish. Take an RCIA class. Learn what the Church teaches.