My road to the Catholic Church began when I realized that what Protestants say about how they discern divinely revealed truth simply doesn’t work. Protestants say that they get this knowledge from the Bible directly, with the immediate (in the sense of unmediated) assistance of the Holy Spirit. The obvious implication of such a principle would be that they should all be in agreement about those things that one must believe in order to be saved. After all, if your Teacher is God Himself, and since God does not lie, how would it be possible for you to disagree with your fellow Protestants about such truths? Of course, there are some things that may reasonably be called matters of indifference, but there are others for which that description is decidedly unreasonable. For example, it is unreasonable to suppose that Protestants can legitimately disagree in their beliefs about God’s nature and the Trinity if God helps them to understand the Bible in the way that they claim. It is unreasonable to suppose that Christ has given them sacraments but that their meaning and number is uncertain if God helps them to understand the Bible in the way that they claim. And yet they most certainly do differ on these questions (and many others). But if they differ on such matters, it means that the Holy Spirit is saying different things to different people—and not just different, but contradictory things. But God does not lie. Therefore it cannot be the case that the Holy Spirit leads them to the truth in the way that they say.
This is not to say that it is impossible to discover truth in the Bible by reading it; of course it is possible. It is to say, rather, that it is not possible to achieve certainty about the truth we might discover there. This would not be the case if the Holy Spirit operated in the way that Protestants claim, because God both knows the truth with certainty and does not lie. These things being so, I realized that I could not remain a Protestant, and in the course of time I and my family were received into full communion with the Catholic Church.
A suitable object lesson concerning the claim that I’ve just made—namely, that the Protestant is wrong about how the Holy Spirit works in relation to the Bible—can be found in the ongoing controversy over the so-called “Federal Vision” (FV) or “New Perspective on Paul” (NPP) among the conservative Reformed denominations. Multiple Presbyterian and Reformed denominations have condemned FV/NPP as theological error; at least one seminary professor has been fired, and pastors and congregations have moved from one denomination to the other as a consequence. The major bone of contention seems to focus on the Protestant idea of sola fide justification. The denominations apparently claim that FV/NPP contradicts this doctrine, and they take it sufficiently seriously as to be perfectly willing to break fellowship with their (former) brethren over it.
Now it ought to be uncontroversial to say that the general question of justification is not a matter of theological indifference: after all, it was a primary dispute (or perhaps the primary dispute) of the Reformation. And yet here we have Reformed folks vehemently disagreeing about it. Considering that they have condemned it as a significant error, we may safely conclude that the majorities in the denominations that have condemned FV/NPP do not believe that the Holy Spirit has led its adherents to their position, but on the contrary has led themselves to the truth. The problem with that supposition is that there is no principled reason to believe anything of the sort. The FV/NPP adherent firmly believes that the Holy Spirit has led him to the truth. On what grounds, then, has the majority determined that this is not true? It should be obvious that there is no objective basis for saying so. They may appeal to Reformed confessional standards, but obviously that doesn’t imply that the FV/NPP is wrong, but simply not Reformed. Now this might be sufficient grounds in their eyes to warrant breaking fellowship, but it’s another thing entirely to say that FV/NPP is wrong or unscriptural, which is the usual associated claim. And the majority simply cannot prove that the Holy Spirit has not spoken to the FV/NPP group: they have no objective, principled grounds for doing so.
Hence we see that—even with respect to questions that they themselves consider to be of utmost importance—Protestant epistemology doesn’t work.
Aquinas, Etc. 
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