When discussing my conversion with a Reformed TA the other day I said at one point in explanation. I don't believe that after Pentecost the defectibility of the Church could be as great as you understand it. If I hadn't studied history I would've much more easily condemned Sts Bede, Brendan, Augustine (the lesser), etc, as ignorant of 'the only necessary thing' and the 'article on which the church stands or falls'. In any case, I'm not trying to attack Protestantism - though it sounds like I am - I'm just trying to offer a bit of an explanation.
As I studied early medieval and anglo-saxon Christianity I began to feel a deep connection to St. Augustine and all those holy men and women who worked to convert the land of my fathers the first time. It was then I began to question: with all the developments of doctrine, what is the single unifying experience of salvation that links Christians through every age. Only after beginning my attendance at the Anglo-Catholic Church in town did I realize it was the Sacraments, God's objective means of grace.
But I notice something in some Anglicans that troubles me. Even as an undergrad I'm going to have the arrogance to call it "Bad History". It's arguments like this that I see:
"The Celtic (even that word is debated by modern historians) church disagreed with Rome over the date of Easter, therefore in England Protestantism always secretly existed".
The underlying problem is that the whole host of "Roman" things done by the Irish/Celtic Church still occured despite the date of Easter. Relics, the Sacrifice of the Mass (which entails non-Protestant soteriology), the REAL REAL presence/transubstantiation, penance, saintly intercession, etc was all a part of "Celtic" Christianity.
It's a very anachronistic view of History and is why in my opinion Catholic (Roman) historiography is the only one that makes sense. But this is not to say that there aren't other good arguments against Catholicism, simply that HISTORY is always in defense of these "Romish" doctrines, as far as I have studied.