Through Joseph's dream, Mary's innocence is confirmed. Mary is to give birth to a human child. The Child would go on to do things no human had ever done (and things that no human will ever do again). In Joseph's dream, the angel of the Lord did not directly state that the Child in Mary's womb was God Himself, but this truth could easily be implied— for what normal human being can save his people from their sins?
After Joseph's dream comes to an end, Matthew interrupts the narrative and states:
Now all this took place to fulfill what was spoken by the Lord through the prophet: "BEHOLD, THE VIRGIN SHALL BE WITH CHILD AND SHALL BEAR A SON, AND THEY SHALL CALL HIS NAME IMMANUEL," which translated means, "GOD WITH US." (Matthew 1:22-23)The prophets had spoken much of the coming of the Messiah. Isaiah especially had much to say concerning the coming of this King. By inserting this quotation from Isaiah 7:14, more credibility is added to Matthew's account of Jesus' birth— for it shows that ancient prophesy was in alignment with the way in which Jesus came into the world.
The contents of Isaiah's prophecy are used to further demonstrate that this Child was no ordinary child. Not only was this Child conceived of by the Holy Spirit, not only was this Child to to be born of the virgin Mary, not only was this Child to save His people from His sins— but this Child was Immanuel, God with us. The Saviour— the Messiah. This Child was God Himself!
Isaiah prophesied of the coming of this Child over five-hundred years before Jesus' birth. This Child was not Plan B. The coming of this Child had been planned when Isaiah walked the earth. The coming of this Messiah had been planned when Abraham offered up Isaac as a sacrifice, for The LORD Will Provide (Genesis 22:14). The coming of this Messiah had been planned at the fall, when God told Satan that Eve's seed would bruise him on the head (Genesis 3:15). The coming of the Messiah had even been planned from before the foundation of the world (Ephesians 1:4-5)!
Isaiah's Prophecy: God With Us (Part 2) - Matthew 1:22-23
Jesus' Purpose: To Save His People From Their Sins (Part 1) - Matthew 1:21
Joseph, Do Not Be Afraid to Take Mary as Your Wife - Matthew 1:20
Joseph's Dilemma - Matthew 1:19
The Virgin Birth - Matthew 1:18