I think I would distinguish between these two internally coherent positions with this observation:
a) someone who does not view Matthew and Luke as Holy Scripture could conclude that the idea that Mary conceived Jesus without sexual intercourse was developed between the time that the Pauline epistles were penned and the time that those gospels were penned.
b) in contrast, unless one is of that attitude that the Church was divinely guided and protected from any error, I don’t know how one would avoid concluding that the “Catholic” Marian doctrines were developed well after the fact from nothing but pious imagination.
First of all, I don’t think infallibility is the issue. Maybe in the first instance, given how difficult the virginal conception is for many modern folks (so that many of us need the infallible authority of the Church telling us that this is a non-negotiable). I would also add that it would be quite possible to hold Matthew and Luke to be Holy Scripture and still interpret the virginal conception as a pious legend–unless one is going to maintain that Holy Scripture contains no pious legends. (I’m being somewhat facetious, since that is probably what you
do believe.) What prevents us from doing so is the historic proclamation of the Church (and to some extent also Luke’s clear intention to write a work of history). In short, the virginal conception may need infallibility for us to believe it because it’s a concrete claim about something that happened, and something that takes considerable faith to believe since it’s not the sort of thing that normally happens and is found with very different details in two Gospels which many scholars would put fairly late in the first century. Similarly, the only way I could believe in the perpetual virginity of Mary (particular virginity in partu) would be to accept it on the authority of the Church. It’s one of the very few things that I hesitate to accept as an Anglican but would have to accept if I finally became Catholic or Orthodox.
But with regard to Marian piety generally, I think that this has much to recommend it on its own ground as a devotional elaboration on Christian belief in the Incarnation.
At any rate, the broader question in both cases really isn’t about infallibility but about whether we take a critical view of doctrinal development or not. What I mean by a critical view in this context is setting up what appear to be earlier stages in the tradition over against later ones. That is what the OP did. But in typical conservative Protestant fashion, the OP drew the line at applying this methodology *within *Scripture.
My challenge is: why? Given that acceptance of Scripture in the first place depends to some extent on accepting the historic witness of the Church, this methodology doesn’t make sense to me.
Your response appears to be that the virginal conception story can stand “on its own” much better than later developments can. Sure, if we are talking about historical claims. In other words, if any of the things that Christians have traditionally claimed about Mary are true, then the virginal conception would be the most likely even on strictly historical grounds, because it dates from earlier texts than the other claims and might conceivably (as I believe, at least in the case of Luke’s narrative) have arisen from the Gospel writers’ inquiries into the circumstances of Jesus’ birth rather than simply from “pious imagination.” Or, to put it more personally, I believe that the virginal conception really happened, even though I need lots of “help” from the authority of the Church to do this. I don’t think I could persuade myself to believe that everything in the Protoevangelion of James really happened as described. That text is quite obviously pious legend.
If we are talking about the relative likelihood of the virginal conception, the perpetual virginity (and other stories about Mary found in the Protoevangelion of James), and the Assumption/Dormition legends being historically accurate, then I certainly agree that each of these stories is progressively much less likely than the one before. But I took the OP to be speaking primarily about
doctrine. And that’s the ground on which I answered. The best explanation for why Mark and Paul don’t talk about the virginal conception, or show much interest in Jesus’ birth at all, is that their theology didn’t require it. Something had changed *theologically *by the time we get to Matthew and Luke (of course it might not be change but just a different perspective held by people who lived at the same time–but since Matthew and Luke do appear to be some decades later, change is quite likely). As Christians came to reflect more deeply on who they believed Jesus to be, they came to see the circumstances of His birth as more important than they had previously thought.
Similarly, language about Mary as the New Eve and the God-Bearer, as well as devotional practices such as making icons of her and asking for her prayers, developed out of a deeper understanding of the mystery of the Incarnation and a cultural and spiritual life based on that mystery.
Edwin