EVERVIRGIN has been a title of our Lady from the earliest days; it appears, albeit obiter, in the documents of councils from Chalcedon onwards. It still appears (confiteor; Communicantes) in the Novus Ordo Mass; was rather more frequent in the Classical Roman Rite; and comes very often in the Byzantine Rite. It is part of the Church's Marian dogma, and was treated respectfully, if rather evasively, by the ARCIC document on Mary. Non-Catholics sneer at it. The great Tom Wright is dismissive. Let us consider the question in the form of a Socratic Dialogue.
The Gospels make it quite clear that Jesus had brothers.
They don't. Adelphoi can mean kinsmen. It doesn't have to mean uterine (that is, born-of-the-same-womb) brothers.
So you say. But that's the obvious meaning if anyone talks about "Jesus' brothers" in any language, isn't it? Not at all. Mark's and Matthew's Gospels, in their accounts of the Crucifixion, both talk about "Mary the mother of James and Joses [or Joseph]". If this Mary had been the same as Christ's own mother, it would have been very odd for them not to refer to her as the Mother of Jesus. The "obvious" and natural inference is that the "Mother of James and Joses" was a different Mary from "Mary the Mother of Jesus".
Well, in Mark 6:3 and Matthew 13:55, the places where those "brothers of Jesus" are mentioned, the full text reads: " Jesus the son of Mary and brother of James and Joses [or Joseph] and Judas and Simon". We've just seen that this James and this Joses are apparently the sons of some Mary who was not the same as Mary the Mother of Jesus. And they're the first two on the list here. The list is thus clearly not itemising individuals who were uterine brothers of Jesus.
Well, I still think it's obvious that ...
If it's so "obvious", you've got some explaining to do. Throughout the second century the Gospels were increasingly regarded as 'canonical' and authoritative. If it is so "obvious" that James and the rest of those listed in the Gospels were uterine brothers of Jesus, then the tradition that Jesus was Mary's only child must have arisen well before those Gospels came to be regarded as authorities. Otherwise, when somebody started saying "she never had any more children", somebody who had read the Gospels would have said "Aha, you're wrong: here's a list of his brothers". So, if you're right about it being so "obvious", you're going to have to admit that Mary's perpetual virginity is so early a tradition as to predate the acquisition of authority by our Four Gospels; which modern scholarship dates to the beginning of the second century at the latest. I've got you either way.
That's all gobbledegook. It's obvious ...
That's the problem with you Prods and you Liberals. You're impervious to evidence and to reason.
Of course we are. "Reason is the Devil's Whore". Martin Luther said so. It's obvious.