3. His first statement was: “Matthew says, [Matthew 1:18-20] Now the birth of Jesus Christ was on this wise: When his mother Mary had been betrothed to Joseph, before they came together she was found with child of the Holy Ghost. And Joseph her husband, being a righteous man, and not willing to make her a public example, was minded to put her away privately. But when he thought on these things, behold, an angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.” “Notice,” he says, “that the word used is betrothed, not intrusted as you say, and of course the only reason why she was betrothed was that she might one day be married. And the Evangelist would not have said before they came together if they were not to come together, for no one would use the phrase before he dined of a man who was not going to dine. Then, again, the angel calls her wife and speaks of her as united to Joseph.” We are next invited to listen to the declaration of Scripture: [Matthew 1:24-25] “And Joseph arose from his sleep, and did as the angel of the Lord commanded him, and took unto him his wife; and knew her not till she had brought forth her son.”Finally, Jerome is getting around to an actual argument, and kudos to Jerome, he is presenting his opponent's position first. Specifically, what Jerome has done here is to present what Helvidius argued, apparently as a quotation from or at least a paraphrase or summary of Helvidius. Considering that Jerome had accused Helvidius of being loquacious, one suspects that this may be a summary of Helvidius' argument, rather than the entirety of it.
Helvidius argued from Matthew 1 that the word used is "betrothed" not merely "entrusted" and the reason for the betrothal was ultimately marriage. Helvidius further argued that Matthew wouldn't have written "before they came together" unless they were going to come together. Likewise, the angel calls Mary Joseph's wife and speaks of her as being united to Joseph, according to Helvidius. Finally, there seems to be implied that "knew her not till ..." implies that Joseph eventually knew Mary.
All of these seem to be sound arguments. To them, we may add the argument we mentioned in the previous section, namely that Joseph was minded to put away Mary, not to seek out her seducer or to return her to her father or elsewhere. Putting away implies divorce, which implies a marriage (of which betrothal is a first step), not merely an entrustment.