You’ve heard of God of the gaps before, but Protestants utilize something I call “Faith of the gaps” to defend their beliefs. And this kind of argument doesn’t work for the same reason that the God of the gaps one doesn’t.
Protestants are not unreasonable. In fact they’re quite reasonable, up until a certain point, and then they have to resort to Faith of the gaps. Here’s how: Take the canon of Scripture. Protestants make historical arguments in support of their canon. Catholics point out how these cannot actually give them their certainty needed to believe their canon is correct, but when faced with these rebuttals, instead of realizing that they have a fundamental problem, one that strikes at the root of their beliefs, they claim that that hole in their reason is where “faith” comes in. You see, “faith” fills in the gaps left by Protestantism’s ad hoc judgments.
For the canon, one ad hoc judgment is the fact that there is no principled reason for believing that God guided the early Church’s discernment of the doctrine of the canon while rejecting the belief that God guided the early Church on other doctrines.
This Faith of the gaps actually sounds reasonable enough. Just listen to what a Protestant would say to himself: “Well, I’ve researched the canon and found many arguments and pieces of evidence for the Protestant one over the Catholic one. They seem reasonable, so I have reason to believe that the Protestant canon is the true one. Also [get ready for Faith of the gaps assurance here] since I have gleaned so much wisdom from the Bible and encountered Christ in it, I know that it is true.”
What’s the problem here? It’s simply that faith builds upon reason, but that reason must be sound reason. Solid philosophical arguments. Principled reasons. Motives of credibility that are accurate. So the edifice of reason supporting Catholicism is sound, but its purpose is limited to being the point where the person makes the assent of faith. Sound reasoning allows one to make the assent of faith seamlessly, as a hang glider gets a running take off from a tall ledge. Faulty reasoning is akin to that hang glider stumbling over rough stones, not getting the momentum needed for a proper take off, and then relying on Faith of the gaps to somehow correct that problem and get him into flight. He may get into some sort of flight in a partial or wobbly way (analogous to, say, a canon missing 7 books that God inspired), but it won’t send him soaring to the highest heights he was created for.
The ultimate cause of this Protestant problem though is not one of reason. Ironically, it is one of faith. Because Protestants reject the Catholic belief that Christ founded a visible Church and has protected His Church from error in her teachings, they are forced to fill in the holes, inconsistencies, and discontinuities in their beliefs with “faith,” yet doing so is really (unintentionally) being fideistic and not the true use of faith.
Hopefully this helps you understand (if you’re Catholic), why Protestants don’t all just become Catholic immediately after they are presented with the problems in the edifice of reason supporting their faith. The Faith of the gaps is an almost irresistible device to turn to.
But God has provided many ways for us to come to know Him in the fullness of the truth in the Catholic Church. One of those is by people carefully learning how to reason and then applying that knowledge to their beliefs. When they see the basis for their beliefs (like the books of the Bible) rests on an ad hoc decision, that can lead them, by God’s grace, to stop and say “that doesn’t quite make sense, hmm…” and lead them deeper into the truth of Christ.