22 So all this was done that it might be fulfilled which was spoken by the Lord through the prophet, saying:
23 “Behold, the virgin shall be with child, and bear a Son, and they shall call His name Immanuel,” which is translated, “God with us.”
24 Then Joseph, being aroused from sleep, did as the angel of the Lord commanded him and took to him his wife,
25 and did not know her till she had brought forth her firstborn Son. And he called His name Jesus.
the prophet (v.22) — Isaiah 7:14
Pentecost and Gaebelein have slightly different takes on the exact meaning of Isaiah’s sign from God.
Matthew called to our attention that fact that this miracle which had been announced to him had been promised to Israel in Isaiah 7:14, the great prophecy of the virgin birth. In the context of that passage, the kingdom of Judah was being threatened by a coalition of Israel and Syria. God sent Isaiah to bring King Ahaz of Judah a message of comfort. Isaiah promised that this coalition would fail and Judah would survive. The message was so important that God offered to confirm this promise to Ahaz. Isaiah asked Ahaz to seek a sign that God would fulfill His promise. However, Ahaz refused to permit God to give him a sign. This was not because Ahaz trusted God but becuase he refused to be obligated to submit to God, for he was a rebel against Him. Isaiah gave Ahaz a sign, nevertheless. It was the sign of the virgin birth. This prophecy had a double reference. The word for “virgin” in Isaiah 7:14 is a broad word that refers to any young woman of marriageable age. The prophecy was intended to convey to Ahaz the promise that before a young woman of marriageable age could be married, conceive, bear a son, and wan that son, Judah would be rid of her enemies. Thus within about three years Ahaz would see the fulfillment of God’s promise to him that he powers allied against Judah would fail. But the prophecy went far beyond the immediate reference to Ahaz, for it was a prophecy concerning the virgin birth of Christ. When the New Testament referred to this prophecy, it selected the restrictive word for a virgin rather than interpreting it in a broader sense to mean simply a young woman. As Matthew wrote and interpreted the prophecy, he told us that Isaiah had in mind the miraculous conception of Jesus in the womb of Mary by the power of the Holy Spirit. The definite article “the” used with virgin shows that Isaiah had one virgin in mind — Mary herself. — Pentecost, pages 54-55.
So that, although that prophecy, of a virgin’s bringing forth a son, should not be fulfilled till many hundreds of years after, yet, at that present time, when the prophecy was made, Ahaz had a certain and notable sign, that the house of David should be safe and secure from the danger that hung over it. As much as if the prophet had said: “Be not so troubled O Ahaz, does it not seem in impossible thing to thee, that never will happen, that a pure virgin become a mother? but I tell thee such a virgin shall bring forth a son, before the house of David perish. — Gaebelein, page 36.
I think Gaebelein’s take makes a little more sense, but either view supports Matthew’s contention that Mary was a pure virgin when she gave birth to Jesus Christ.
Joseph’s response (v.24) was one of implicit faith and obedience. He did not ask for confirmation. He did not ask for explanation. He accepted the fact that Isaiah’s prophecy concerning the virgin birth was the true explanation of Mary’s pregnancy, and he obeyed the command of the angel and “took Mary home as his wife” (Matthew 1:24). Joseph demonstrated a remarkable restraint in that “he had no union with her until she gave birth to a son” (v.25). Thus Christ was truly born of a virgin … By the most natural interpretation of the expression “first-born,” Mary had other children, which were born from the wedlock of Joseph. The Gospel narratives speak of four such younger brothers and at least two sister. — Pentecost, page 55.