Matt 1:25 And he knew her not till she brought forth her firstborn son: and he called his name Jesus.
St. Jerome shews, by divers examples, that this expression of the Evangelist was a manner of speaking usual among the Hebrews, to denote by the word until, only what is done, without any regard to the future.
Thus it is said, Genesis 8. 6 and 7, that Noe sent forth a raven, which went forth, and did not return till the waters were dried up on the earth. That is, did not return any more.
Also Isaias 46. 4, God says: I am till you grow old. Who dare infer that God should then cease to be?
Also in the first book of Machabees 5. 54, And they went up to mount Sion with joy and gladness, and offered holocausts, because not one of them was slain till they had returned in peace. That is, not one was slain before or after they had returned.
God saith to his divine Son: Sit on my right hand till I make thy enemies thy footstool. [Ps 110:1] Shall he sit no longer after his enemies are subdued? Yea and for all eternity.
St. Jerome also proves by Scripture examples, that an only begotten son, was also called firstborn, or first begotten: because according to the law, the firstborn males were to be consecrated to God: Sanctify unto me, saith the Lord, every firstborn that openeth the womb among the children of Israel, etc. [Exod 13:2]
Scripture and note from the Douay-Rheims Bible, Challoner Revision (New York: P. J. Kennedy & Sons, 1914).