Then Joseph her husband, being a just man, and not willing to make her a public example, was minded to put her away privily.

But while he thought on these things, behold, the 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….

Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife:

And knew her not till she had brought forth her firstborn son: and he called his name JESUS.

The text sets forth the only possible way in which Jesus Christ could be our Savior. He was conceived, not by man, but by the Holy Ghost. Thus He was truly God and at the same time sinless, righteous man. Born in any other way would have made His being our Savior impossible.