Early in Luke's gospel (Luke 1:26-38) we read about an angel announcing to a young woman, a virgin named Mary, that she will give birth to a baby boy named Jesus. But let's backtrack. 

This woman, Mary, was likely so young that most of us would prefer to call her a girl. She was likely in her early teens when the angel appeared to give her this news. She'd lived her whole life (12, 13, 14, 15 years?) years in a tiny, backwater village named Nazareth in a rural region called Galilee, in an insignificant portion of the Roman Empire called Israel. She was not the cream of the crop, so to speak. She was not a well-educated, affluent, young woman from an influential family, with many potential career opportunities before her. Instead she was, in earthly terms, a nobody. Historical records would have left her in obscurity, but for her son.

...but for her son.

Her son was her son. But her son was also God's Son. His name was to be called Jesus, because he would save his people from their sins (Matthew 1:21). He would save Mary, herself, from her sins (Luke 1:47). Though Mary was a nobody, her son was not a nobody. He was a somebody. He was the Creator, the Lord of the cosmos, God-eternal become flesh. Jesus was the Saviour of his people, Jew and Gentile, all who would come to believe in him.

And Jesus would be the Saviour from sin by becoming the subsititute for his people-- the propitiating (wrath-diverting) sacrifice for sin, the sin offering, the atonement... the Lamb.

In Luke 2:22-24 we read that Mary came to the temple, with Jesus, "to offer a sacrifice according to what is said in the Law of the Lord, 'a pair of turtledoves, or two young pigeons.'" This quote comes from Leviticus 12:6-8 where the Law prescribes that, "...if she cannot afford a lamb, then she shall take two turtledoves or two pigeons..." So Mary could not afford a lamb.

In view of this, what a beautiful thing God did for this girl who could not afford a lamb! He gave her One free of charge. The girl who could not afford a lamb was given a Lamb as a Christmas present so many years ago. The girl who could not afford a lamb was graciously chosen by God to give birth to "the Lamb of God, who takes away the sin of the world!" (John 1:29)

Perhaps God chose Mary to be the mother of Jesus in order to show how his grace works. It is not something we can afford. It is not something we merit. It is not due to some intrinsic worthiness on our part. And it is not something we pay back, after receiving. Rather, it comes to those who are destitute and needy. It comes to those who are insignificant and unworthy. From beginning to end, as Ephesians 2:8-9 tell us, "it is the gift of God."