I've been dating some Catholics on Facebook. A few of my comments:
1. I'd like to follow up Edwin's query about sufficient evidence.
i) To begin with, I wasn't making a case for the Protestant faith, but presenting an argumentative strategy. How to frame the argument.
ii) I haven't given Edwin a detailed answer because there is no one answer. What counts as evidence depends on the nature of the claim. Different kinds of claims require different kinds of evidence. Scientific evidence may be different from historical evidence (although they sometimes overlap). Evidence for abstract objects is different from evidence for concrete objects. To take a few Marian examples:
iii) What would count as evidence for the Immaculate Conception? What kind of evidence would even be probative? In the nature of the case, there could be no physical evidence for the Immaculate Conception.
In what respect, if at all, would the Immaculate Conception even be detectable to Mary or her parents?
Assuming (ex hypothesi) that Mary was sinless, what evidence could there be that she was sinless from the moment of her conception, rather than at some later stage in utero, or as a newborn baby, or one-year-old? In other words, if God intervened to exempt her from the stain of original sin, how would Mary or her parents know when that happened? Even in principle, how could there be any evidence for the timing of God's intervention?
For that matter, surely the church fathers had a different understanding of conception and the moment of conception, than we do, thanks to modern gynecology and related disciplines. So what does the traditional dogma even mean?
And assuming (ex hypothesi) that there was some sort of evidence which the concerned parties could recognize, how does it follow that church fathers were privy to that? Is it not far more likely that this belief evolved through multiple stages of theological speculation?
iv) Let's take a comparison: The external evidence for the traditional authorship of John's Gospel includes the claim that Irenaeus knew Polycarp, who, in turn, knew the Apostle John. That gives us a stated chain of custody. We know who the links are, and how many links there are. And if the report is true, then that would be directly germane to the nature of the claim. In other words, we can see in principle how that information could be reliably transmitted down the line. At least we're in a position to evaluate the evidence.
What do we have that's the least bit comparable respecting the Immaculate Conception, Assumption, and Perpetual Virginity (including in partu virginity)?
v) Consider the virginity in partu. What would even count as evidence for that claim? Would there be physical evidence?
When the hymen is ruptured during initial sexual intercourse, there can be some discomfort or bleeding. If, however, Mary was a virgin at the time of birth, then surely that evidence would be masked or obliterated by the messy, painful process of giving birth. How could Mary, or a midwife, or Joseph (if he had to deliver the child) distinguish the effects of the birthing process from the effects of a ruptured hymen, or an intact hymen?
Keep in mind, too, that Mary didn't give birth in a brightly lit operating room, but in a dimly lit hut.
And even assuming that her hymen remained intact during the process of birth, how would church fathers be privy to that fact? Did Mary go around telling relatives that her hymen was intact, which was somehow passed around by word-of-mouth in Christian circles?
2. Notice two clashing Catholic paradigms. On the one hand is the old, pre-Newmanesque, Counter-Reformation (a la Bellarmine) paradigm, where you attempt to prooftext Catholic dogma from Scripture. On the other hand, you have the theory of development. What is more, Catholic apologists and their acolytes keep assuring us that dogma doesn't require Biblical justification, since Sacred Tradition is another valid source of dogma.
Yet so many Catholics fight tooth-n-nail for these traditional prooftexts, as if they really believe in sola scripture, which makes them cling for dear life some Biblical warrant for each and every Catholic dogma. The schizophrenia is something to behold. Perhaps psychotropic medication will relieve the unbearable tension.
3. Notice that defending Catholicism is just as complicated as defending Protestantism. In my experience over the years, evangelical converts to Catholicism typically swim the Tiber because they imagine that's a simplifying device. That gives the a level of certainty lacking in Protestantism. Now they have a single arbiter to make decisions for them on the interpretation of Scripture, orthodoxy, and orthopraxy.
But the simplification and certainty is illusory. Before they convert, they must convince themselves that Catholic prooftexts from Scripture and church fathers in fact point Romeward. At this preliminary stage of the evaluation, they must exercise their private judgment when they interpret the documentary evidence. Likewise, they must review the historical record and judge for themselves that prima facie reversals in Catholic theology are actually consistent. By the same token, they must somehow determine that apostolic succession is consistent with the Western schism. Those are just a few examples. All the same historical uncertainties and "interpretive pluralism" will confront them at that stage of the analysis.
Put another way, what's the difference between a Protestant apologist and a Catholic apologist? A convert to Rome has merely moved one chair over on the roundtable. If a Protestant apologist must defend his position against objections by Catholics, Orthodox, atheists, and non-Christian religions, a Catholic apologist must defend his position against objections by Protestants, Orthodox, atheists, and non-Christian religions. Both Catholic and Protestant apologists are fighting on multiple fronts to vindicate their respective positions.
Just look at poor little sweaty overworked Mark Daviau. He's like the lone volunteer at a leaky dam. As he's plugging one leak, the dam springs another leak. When he tries to plug the second leak, the seal on the first leak breaks. Or the dam springs a third leak.
Look at how Mark Daviau is constantly on the defensive. The moment his back is turned, the dam springs another leak.
Honestly, is his position any simpler than a Protestant apologist? Indeed, Mark Daviau acts like a gerbil sharing a terrarium with a boa constrictor. Mark dare not close his eyes. The fatal strike could come from any direction at any time.