Just out of interest I thought I would try to write the strongest argument in favour of Confessional Protestantism, and see if I couldn't knock it down later. It is not that I am tempted to believe it, I just want to lay it down for my own understanding.
Faith and Reason:
The great Swiss Reformed theologian Karl Barth argued that natural theology and the use of philosophy in Christian faith, was 'where things all went wrong'. As Luther sayeth, 'the Theologian must first put to death the philosopher (Aristotle)', before burning the Summa Theologiae in public. Barth cites the famous unfinished symphonies as examples, surely no one can listen to the first half, and then using natural reason, discover the remainder, given any amount of time. This he proposed is what Roman Catholics do when they enter theology. Genesis 3:15 says there would be enmity between 'the woman' (Virgin Mary) and Satan. Examine that long enough, and you start talking about the Immaculate Conception, humans as Barth noted are "idol factories", and any attempt to add to the word of God with human words, no matter what the phrasing is 'development', 'unwritten traditions', etc, is sin.
Soren Kierkegaard in "Fear and Trembling" describes the divine command for Abraham to kill his son. Nothing in natural reasoning could explain this as logical. God is not a Being to be reasoned about, he is a speaking God who is to be trusted or rejected.
Roman Catholics ask how Protestants know which Scriptures are valid and which are not, as the Bible did not come straight from Heaven. Granted Barth would agree, BUT as the second Helvetic Confession declares:
"The preaching of the Word of God is the Word of God"
All throughout the book of Acts, it is the declaration of the Word (invisibly in preaching, visibly in the sacraments) that leads to salvation. As it is written, "faith cometh by hearing" (Rom 10:17). While the succession of Bishops may or may not be valid (Anglicanism v. Presbyterianism), the real succession of the Apostles was their preaching. The church is described as witnesses to the resurrection, to the Christ Event, to something that had happened and was now finished, all that was required now was the preaching of it to others. By this message which was preached since the apostles, did the council fathers know what was canonical and what was not. (I could then develop the argument that the new testament was clearly agreed on, Romans says 'to the Jews were entrusted the oracles of God' as the prooftext for the 39 book proto-canon).
In the same way that Luigi Giussani argued that people today can encounter Christ only through Christ's body as an objective historical reality, so would Barth argue that people today are faced to encounter the preaching of the gospel as an objective historical reality.
Furthermore, God has no need of an institution to tell people what the Word of God is, and what it isn't, for Christ is "the true Light, which lighteth every man that cometh into the world." (Jn 1:9), and because the apostle says:
"When God made a promise to Abraham, because he had no one greater by whom to swear, he swore by himself, saying, ‘I will surely bless you and multiply you.’ And thus Abraham, having patiently endured, obtained the promise. Human beings, of course, swear by someone greater than themselves, and an oath given as confirmation puts an end to all dispute. In the same way, when God desired to show even more clearly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it by an oath" -Hebrews 6:13-17
If God swears Scripture by the Church then he is swearing it by human beings. As God has always done - since Abraham - he swears his covenant by himself. The gospel, as Barth notes, is not modified or evaluated by man, it can only be responded to. It is a message which is preached, it is an existential experience and an encounter.
Christ says "If ye continue in my word, then are ye my disciples indeed" (Jn 8:31). For "Heaven and earth shall pass away, but my words shall not pass away." (Mt. 24:35), and so by the direct command of God we must "destroy arguments and every lofty opinion raised against the knowledge of God, and take every thought captive to obey Christ" (2 Cor 10:5). Whatever we can discern through history or tradition must be subject to Christ, for he declares: "the scripture cannot be broken" (Jn 10:35). Scripture makes a clear dichotemy between the Word of God and the words of men, and such a dichotemy allows for tradition to be subjected.
Once they get sola scriptura/prima scriptura, they can then go on to quote Jer. 23:6 "The LORD is our righteousness" and make the imputation of Christ's righteousness argument, add Romans 4:5, and nail down Sola Fide.
Philosophical Realism, Appeals to Church History, Claims of Anachronism, Attacks on Fideism, Appeals to Church Unity, etc. Standard Catholic attacks.
For instance, one could say this argument gives me no case against the Quran as the Word of God beyond: I don't feel that it is. It is basically just a fideist argument that defends irrational trust in a source based on the claim that it is from God. If it is from God (which cannot be determined except existentially/experientially) then it works as an argument, but if more than one faith claims true existential religious experience, then this has to be explained away somehow (usually be anti-Islamic attitudes or racism).
The fact that St. Paul also uses philosophy in the Areopagus in Athens to debate the pagans also destroys this nominalist/Lutheran division of faith and reason.
I'm reading a great book by Von Balthasar about the Petrine office, and he has some of the strongest biblical -and unique ones- arguments for the papacy, and once I'm done that book, I'll post the counter arguments to this thesis.
Even one of our Popes declared Karl Barth to be the most important theologians since St. Thomas Aquinas, so I figured I would set up the strongest argument for Reformed Protestantism using his emphasis.
Protestants reading this: let me know where it should be strengthened.
Catholics reading this: let me know problems with the argument.