‘Until’ she gave birth to her firstborn son by Jerome

“He did not know her until she gave birth to her firstborn Son.” Based on this passage, some perversely suppose that Mary had other children, maintaining that a son is not called “firstborn” unless he has siblings. To the contrary, the divine Scriptures are accustomed to call someone “firstborn”., not because other siblings come after him, but because he is born first.”

Jerome, In Evangelium Matthaei 1, 1, 25; PL 26, 26