When we read the Bible we have spontaneous belief formation that what we are reading is God's word. This is what Christian Philosopher Alvin Plantinga calls a "basic belief". Such beliefs have warrant if they produced by cognitive faculties functioning properly in a truth conducive environment with a design plan aimed at the production of true beliefs. Now this particular epistemological view point is externalistic and is pretty contentious view point since the majority of epistemologists are internalist. But despite that fact it seems to me to be the most reasonable alternative since internalistic epistemologists have failed to give sufficient internal grounds for such beliefs as the external world, our memory being reliable, and induction.
Since it seems reasonable to hold to externalistic proper functionalism then we can apply such notions to the reading of scripture. When we read the Bible we often have a belief form in us that makes us think that God is speaking to us in these documents and in the absence of a reason to doubt this we are warranted in thinking that the Old and New Testament is God's word.
Oddly enough Catholics and Eastern Orthodox think that it is a epistemic virtue in it's own right that we have warrant for the canon. This is when their canon argument comes into play. The East or the West will say "you can't justify the canon and this is a problem for your Protestant view point." But if we could be so sly as to turn the tables on them and ask "Why would we want to do such a thing as that?" If the Bible is not already properly basic and there is no reason for thinking it to be God's revelation then why do we need to justify it in the first place. This is because I think the East and the West are presupposing the Protestant view point because they themselves have such a belief formation through the cognitive faculty of the Holy Spirit when they read Holy Scripture. This is why they feel they ought to have an epistemological framework to justify scripture because it is an already had particular of knowledge.
Finally, if this is all true and it seems reasonable to think that it is, then why do we need an authoritative church speaking for God to justify canon? If it is already properly basic and we have good grounds for thinking it true then we don't need to appeal to a church at all. Why are we positing entities beyond necessity? This does seem to be a violation of a basic principle in metaphysics called "Ockham's Razor". This principle says that we shouldn't posit entities beyond necessity. But this is of course this is what the Roman and Eastern church does. So in light of these consideration's it seems that the Protestant position is once again the most reasonable.