A handy little book for folks kicking at the historic faith of Catholicism is Patrick Madrid's book of the same name. It does a pretty good job of helping those who have spent their lives hearing that Catholicism is based on many things, but not the Bible.
For me, it was the title that caught my attention. For that is the question I, like so many Protestant clergy converts, asked about that most precious of all Protestant dogmas: Sola Scripture. That's the Protestant teaching that the Bible Alone (that's the Sola Scriptura) is the source and authority for faith. While there can be outside traditions, outside interpretative tools, they all spring from the pages of Holy Scripture. Nothing can originate outside of Scripture. It has to have as its source the pages of the inspirited texts; anything originating outside those pages is to be rejected.
Problem. The Bible itself never says that the Bible itself is the sole and only source and authority for the faith. Oh sure, there is plenty Word of God this, and the Word of God that. There are passages that speak of the importance of Scripture, its role in the life of the believer, its importance for instructing, rebuking, correcting and training in righteousness. It was profitable, a lamp unto feet, and quoted by Jesus Himself. But that slam dunk verse, that 'Yea, did the Lord say 'thus shall it be that the written Word of the Lord shall alone be the source of doctrine, teaching, and authority for the faithful' just wasn't there. To get there, I had to have someone tell me it was. Tell me that Revelation 22 was about the entire Canon of Scripture, not just about the Revelation of John. Tell me that profitable, as in 2 Timothy 3:16, meant complete and entirely sufficient and the source of all revealed authority, not merely profitable.
In short, it was a contradiction. Evangelicals and others with a brain can usually have great sport at things like 'there are no absolute truths.' The obvious problem is good for a laugh and a jest. But the problem for me was the same for the Bible. The Bible MUST say somewhere in its pages, clearly and without the need or demand for outside interpreters, that it and it alone is the only source of authority for the faithful. If not, the illogical loop of logical contradictions kicked in.
I discovered through Patrick Madrid's book just how many Catholic doctrines have their roots in Scripture. But more troubling for me, c. 1999, was realizing that there was at least one crucial doctrine upon which all Protestantism had been established, that did not have its roots in a clear teaching of Scripture at all.