Matthew 1:22

Now all this came to pass, that it might be fulfilled which was spoken by the Lord through the prophet, saying, Behold the virgin shall be with child, and shall bring forth a son, and they shall call his name Immanuel; which is, being interpreted, God with us.

The question of whether Isaiah (Isaiah 7:14) fully understood this as applying to the virgin birth of Christ is irrelevant. God's great prophets did not always know the true meaning of the words God gave them. Peter did not know the full meaning of what he prophesied on Pentecost (Acts 2:38,39); and a miracle was required later (Acts 10) to convince Peter that the Gentiles should be permitted entry into the church. See 1 Peter 1:11,12. In this verse, Matthew uses for the first time an expression found ten times in his gospel and nowhere else in the New Testament, "that it might be fulfilled, etc." That the virgin birth is clearly included in Isaiah's prophecy is certain. Matthew declares it IN. The fact that the rabbis and Pharisees had overlooked it is only an indication of spiritual blindness on their part. This beautiful prophecy not only reveals the virgin birth but also sets forth the dual nature of Christ. His name means "God with us!" but his diet is that of a man, "butter and honey"; Here, then, is the GOD-MAN in prophecy!