There is currently a rather large discussion between Protestants and RCs over at GreenBaggins which has led to an opportunity for me to briefly note and rebut two of the more common RC arguments against Protestantism:
1. The first argument might be called "the argument from disunity." Generally, the RC claims that Protestantism leads to disunity; since the body of Christ is [intended to be] united, Protestantism cannot be true.
Reply: it should be apparent that the fact the RC is wagging a finger at so-called Protestantism undermines his argument. The RC isn't attacking a decentralized body; on the contrary, there must be a principle of unity by which the RC is able to identify that which he is attacking - Protestantism - and whatever that principle of unity is, even if it is a caricature, it undermines the claim that Protestantism leads to disunity.
2. The second argument is, essentially, as follows: "Protestants can't justify the canon they believe, because Protestants don't have an infallible Magisterium by which they are able to identify the canon."
Reply: I have recently noted the double standard the RC must here employ, since whatever the nature of evidence to which he appeals to identify the allegedly infallible Magisterium can at least be similarly - and often times "a fortiori" - used as the means by which the canon of Scripture is identified. I have also defended the Protestants capability to recognize the canon here, here, here, and here.
Simply put, because divine revelation must be the justificatory basis of all knowledge-claims (cf. here), God's word must be self-authenticating. If it were not, skepticism would be the inevitable, self-defeating result. This follows because any proposition which would purport to justify God's word as such would presuppose that it (the proposition) has been divinely revealed.
As I pointed out over at GreenBaggins:
//If OT saints were, without the justification of an infallible Magisterium, capable of discerning God’s commandments, why shouldn’t we who possess God’s written word be able to do the same? Was it unreasonable for Abraham to believe that God rather than Descartes’ omnipotent demon commanded that Isaac be sacrificed?
It seems to me that Scripture must be viewed as sub-revelation. God was as able to communicate who He was to the patriarchs without any need for mediation as was Jesus to the apostles (Hebrews 1:1-2). But the RC seemingly believes that the God-breathed Scriptures are not likewise self-authenticating. This is special pleading.//