My conversion to the Catholic Church is surely not unusual in that one of the initial goads I suffered was a realization that "sola scriptura" is an invalid principle by which to hope to learn the truth of divine revelation. Aside from the problem of uncertainty, which is an inescapable concomitant of sola scriptura (well, either uncertainty or a terrible dilution of any realistic sense of the unity of the truth), something I think I've come to understand a bit more clearly is that sola scriptura is an intrinsically humanistic principle. Consequently it is fundamentally alien to Christianity. Or so it seems to me.
In saying this I do not mean to suggest that Protestants are non-Christians. Let's get that out of the way now. I do not think that, nor have I ever thought it. But it's one thing to be Christian by virtue of one's Baptism (which is, of course, that which constitutes one as a Christian) and another thing entirely to subscribe to the Faith as it has been passed down through the ages. It's no secret that Protestant belief is marked by errors that have been condemned by the Church, but this does not mean that properly baptized Protestants are not part of the Body of Christ. They certainly are.
Furthermore, it should be said that Protestants believe the Truth about a variety of things. So when I say that sola scriptura is a humanistic principle, this should not be construed to mean that I think Protestants' use of it always and everywhere results in them arriving at false results. Notwithstanding these caveats, it still remains the case that sola scriptura is not a fundamentally Christian principle, as far as I can see.
The reason for this is that sola scriptura necessarily implies that some human being is resorting to the Bible and making judgments about what it teaches: man is making himself the measure of the truth in God's Word; he is deciding for himself what the content of divine revelation is. This is intrinsically humanistic.
It should not be surprising that this is so. Protestantism arose during the Renaissance. How could it be that Protestantism, in rejecting the authority of the Church, was somehow not doing so thanks to the humanistic influences of the age? It's unreasonable to think that Luther & Co. successfully separated themselves from the spirit of the age entirely. Luther was an Ockhamist as even Bainton admitted. And as I observed in that post, it's not credible to suggest that Luther's training did not influence his theology.