Actually,** we know**. We know because Sacred Tradition tells us, and the Deposit of Faith is not made of Scriptrue alone.
We know also from reason alone: by the very question of Our Lady it is obvious that she intended to preserve her virginity even after her marriage (otherwise the answer to “How shall this come to be?” would have been quite obvious: in a most natural union with her future husband Joseph). Obviously Joseph knew about this, for he would not enter into the marital bound only to find out later that his wife had made a vow to the Lord, and she would not have hidden this from her future husband.
All else is beautiful historical speculation, and it is nice to learn that some thing seemed to be extraneous to the culture of the time, thus rendering her vow quite unique…but in studying the history of Christianity we must keep in mind the warning of the holy pontiff Pius X in Lamentabili Sane and Pascendi Dominici Gregis where he wrote:
With truly lamentable results, our age, casting aside all restraint in its search for the ultimate causes of things, frequently pursues novelties so ardently that it rejects the legacy of the human race. Thus it falls into very serious errors, which are even more serious when they concern sacred authority, the interpretation of Sacred Scripture, and the principal mysteries of Faith.
we have that distinction, so current among the Modernists, between the Christ of history and the Christ of faith; the Church of history and the Church of faith; the sacraments of history and the sacraments of faith, and so in similar matters.
Next we find that the human element itself, which the historian has to work on, as it appears in the documents, is to be considered as having been transfigured by faith, that is to say, raised above its historical conditions.
It becomes necessary, therefore, to eliminate also the accretions which faith has added, to relegate them to faith itself and to the history of faith.
Thus, when treating of Christ, the historian must set aside all that surpasses man in his natural condition, according to what psychology tells us of him, or according to what we gather from the place and period of his existence.
Finally, they require, by virtue of the third principle, that even those things which are not outside the sphere of history should pass through the sieve, excluding all and relegating to faith everything which, in their judgment, is not in harmony with what they call the logic of facts or not in character with the persons of whom they are predicated.
Thus, they will not allow that Christ ever uttered those things which do not seem to be within the capacity of the multitudes that listened to Him.
Hence they delete from His real history and transfer to faith all the allegories found in His discourses. We may peradventure inquire on what principle they make these divisions? Their reply is that they argue from the character of the man, from his condition of life, from his education, from the complexus of the circumstances under which the facts took place;
As history takes its conclusions from philosophy, so too criticism takes its conclusions from history. The critic on the data furnished him by the historian, makes two parts of all his documents. Those that remain after the triple elimination above described go to form the real history; the rest is attributed to the history of the faith or, as it is styled, to internal history.
For the Modernists distinguish very carefully between these two kinds of history, and it is to be noted that they oppose the history of the faith to real history precisely as real.
Thus, as we have already said, we have a twofold Christ: a real Christ, and a Christ, the one of faith, who never really existed; a Christ who has lived at a given time and in a given place, and a Christ who never lived outside the pious meditations of the believer – the Christ, for instance, whom we find in the Gospel of St. John, which, according to them, is mere meditation from beginning to end.
The Modernists have no hesitation in affirming generally that these books, and especially the Pentateuch and the first three Gospels, have been gradually formed from a primitive brief narration, by additions, by interpolations of theological or allegorical interpretations, or parts introduced only for the purpose of joining different passages together.
Ideed, this history they actually do write, and with such an easy assurance that one might believe them to have seen with their own eyes the writers at work through the ages amplifying the Sacred Books. To aid them in this they call to their assistance that branch of criticism which they call textual, and labor to show that such a fact or such a phrase is not in its right place, adducing other arguments of the same kind. They seem, in fact, to have constructed for themselves certain types of narration and discourses, upon which they base their assured verdict as to whether a thing is or is not out of place.
The truth is that a whole multitude of Doctors, far superior to them in genius, in erudition, in sanctity, have sifted the Sacred Books in every way, and so far from finding in them anything blameworthy have thanked God more and more heartily the more deeply they have gone into them, for His divine bounty in having vouchsafed to speak thus to men.
While the Holy Father is here speaking on much higher issues and concerning very grave errors, we must keep in mind the basic principles he is teaching as part of his ordinary magisterium in order to see where does historical research step out of bounds.