Davidic lineage, adopts the son of Mary. Ironically through this son, anyone uniting with Christ is adopted by God.
The story reveals a scandalous act leaving Joseph and Mary's community to wonder... "why Joseph would continue his relationship with a suspicious woman and how it could be that Mary's child was begotten by the Holy Spirit." It all seemed so curious, suspicious. In an honor/shame culture, such a scandal dishonored not only their own reputations but those of their family. What courage it took to participate with God at the expense of reputation and social stigma. The young couple separated to God... boldly following God and participating with him in bringing life to the world--even though it may have cost them a great deal of social respect.
This story reveals an amazing conception in which God joins with humanity uniting humans with himself in reconciliation. I am reminded of Isaiah who said of God, "I looked for a righteous man who would stand in the gap but found no one, so my own arm worked salvation." Christ is the man able to fill the gap between God and humans. He is the God who added to himself humanity and dwelt among us--coming as a vulnerable child. How amazing is this conception... and how amazing is the picture of reconciliation it gives.
Finally the story reveals the one who would be present... the one named Emmanuel... God who is with us. God is with us in presence and in power. A new relation with God that empowers humans to live in the way of Jesus.