21. But as we do not deny what is written, so we do reject what is not written. We believe that God was born of the Virgin, because we read it. That Mary was married after she brought forth, we do not believe, because we do not read it.http://www.newadvent.org/fathers/3007.htm
He also makes a case for sola scriptura as well though our Catholic friends will not see it.
Bear in mind Jerome was born ~347 AD and died 420 AD. He is believed to have been baptized around ~360-366 AD....~260 yrs after the last apostle died
This tract appeared about a.d. 383. The question which gave occasion to it was whether the Mother of our Lord remained a Virgin after His birth. Helvidius maintained that the mention in the Gospels of the "sisters" and "brethren" of our Lord was proof that the Blessed Virgin had subsequent issue, and he supported his opinion by the writings of Tertullian and Victorinus. The outcome of his views was that virginity was ranked below matrimony. Jerome vigorously takes the other side, and tries to prove that the "sisters" and "brethren" spoken of, were either children of Joseph by a former marriage, or first cousins, children of the sister of the Virgin. A detailed account of the controversy will be found in Farrar's "Early Days of Christianity," pp. 124 sq. When Jerome wrote this treatise both he and Helvidius were at Rome, and Damasus was Pope. The only contemporary notice preserved of Helvidius is that by Jerome in the following pages.http://www.newadvent.org/fathers/3007.htm
There are no known surviving works of Helvidius. It is hard to accept only a one sided argument as absolute truth.
Peak years of paganism entering and being incorporated into the Roman Catholic church.
So Jerome wants us to believe that Joseph disobeyed the angel and did not marry Mary and that she and Joseph lived together with the appearance of being husband and wife, thus deceiving everyone about their marital status.
Jerome misses the fact that Mary and Joseph were already married at the time of the Anunciation.