So the first promise of a son is a promise of a savior. God said to the serpent:

I will put enmity between you and the woman,
and between your offspring and her offspring;
he shall bruise your head,
and you shall bruise his heel.

This was important to Eve. She thought her first son might be the fulfillment of the promise: “Now Adam knew Eve his wife, and she conceived and bore Cain, saying, ‘I have gotten a man with the help of the Lord.'” But she struggled. First son exiled for murder. Second son killed by his brother. Finally a third:

And Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him.” To Seth also a son was born, and he called his name Enosh. At that time people began to call upon the name of the Lord.

Jump ahead to Abraham. The promise of a son with the promise of victory over the enemy is repeated:

After these things the word of the LORD came to Abram in a vision: “Fear not, Abram, I am your shield; your reward shall be very great.” But Abram said, “O LORD God, what will you give me, for I continue childless, and the heir of my house is Eliezer of Damascus?” And Abram said, “Behold, you have given me no offspring, and a member of my household will be my heir.” And behold, the word of the LORD came to him: “This man shall not be your heir; your very own son shall be your heir.” And he brought him outside and said, “Look toward heaven, and number the stars, if you are able to number them.” Then he said to him, “So shall your offspring be.” And he believed the LORD, and he counted it to him as righteousness.

So Abraham believing God’s word and being justified by faith is found in his believing the promise from God that He will provide a son. And in this promise God goes on to make a covenant with Abraham, a promise which includes the defeat of enemies. In fact, this victory is described in judicial language. God will condemn the new “serpent” and thus vindicate or justify his people:

Know for certain that your offspring will be sojourners in a land that is not theirs and will be servants there, and they will be afflicted for four hundred years. But I will bring judgment on the nation that they serve, and afterward they shall come out with great possessions. As for yourself, you shall go to your fathers in peace; you shall be buried in a good old age. And they shall come back here in the fourth generation, for the iniquity of the Amorites is not yet complete.

When God fulfilled this promise in the Exodus from Egypt he interpreted it as this passing of judicial sentence, saying, “For I will pass through the land of Egypt that night, and I will strike all the firstborn in the land of Egypt, both man and beast; and on all the gods of Egypt I will execute judgments: I am the LORD” (Exodus 12.12). And again:

On the day after the Passover, the people of Israel went out triumphantly in the sight of all the Egyptians, while the Egyptians were burying all their firstborn, whom the Lord had struck down among them. On their gods also the Lord executed judgments (Numbers 33.3b-4).

So when God promises Hannah the barren second wife of Elkanah a son, she sees this as the portent of a world revolution in her society:

My heart exults in the Lord;
my strength is exalted in the Lord.
My mouth derides my enemies,
because I rejoice in your salvation.

There is none holy like the Lord;
there is none besides you;
there is no rock like our God.
Talk no more so very proudly,
let not arrogance come from your mouth;
for the Lord is a God of knowledge,
and by him actions are weighed.
The bows of the mighty are broken,
but the feeble bind on strength.
Those who were full have hired themselves out for bread,
but those who were hungry have ceased to hunger.
The barren has borne seven,
but she who has many children is forlorn.
The Lord kills and brings to life;
he brings down to Sheol and raises up.
The Lord makes poor and makes rich;
he brings low and he exalts.
He raises up the poor from the dust;
he lifts the needy from the ash heap
to make them sit with princes
and inherit a seat of honor.
For the pillars of the earth are the Lord’s,
and on them he has set the world.

He will guard the feet of his faithful ones,
but the wicked shall be cut off in darkness,
for not by might shall a man prevail.
The adversaries of the Lord shall be broken to pieces;
against them he will thunder in heaven.
The Lord will judge the ends of the earth;
he will give strength to his king
and exalt the power of his anointed.

Mary too follows in this tradition. Since she is told that her son will be a king, her song is not as surprising to us. But given the precedent of Hannah’s song, Mary might have sung much of it as she did without such knowledge:

My soul magnifies the Lord,
and my spirit rejoices in God my Savior,
for he has looked on the humble estate of his servant.
For behold, from now on all generations will call me blessed;
for he who is mighty has done great things for me,
and holy is his name.
And his mercy is for those who fear him
from generation to generation.
He has shown strength with his arm;
he has scattered the proud in the thoughts of their hearts;
he has brought down the mighty from their thrones
and exalted those of humble estate;
he has filled the hungry with good things,
and the rich he has sent away empty.
He has helped his servant Israel,
in remembrance of his mercy,
as he spoke to our fathers,
to Abraham and to his offspring forever.

The last part of Mary’s song recalls Abraham’s own worries about having a child and the implications of God’s promise to him. Abraham is promises a son and deliverance from the powers. Mary is promised a son and knows it means deliverance from the powers. Abraham’s faith is counted as righteousness. Mary is blessed in her faith, as Elizabeth says: “blessed is she who believed that there would be a fulfillment of what was spoken to her from the Lord.”

And Paul agrees that this is all about justification by faith:

For the promise to Abraham and his offspring that he would be heir of the world did not come through the law but through the righteousness of faith. For if it is those of the law who are to be the heirs, faith is null and the promise is void. For the law brings wrath, but where there is no law there is no transgression.

That is why it is of faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the one of the law but also to the one who shares the faith of Abraham, who is the father of us all, as it is written, “I have made you the father of many nations”—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist. In hope he believed against hope, that he should become the father of many nations, as he had been told, “So shall your offspring be.” He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah’s womb. No distrust made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised. That is why his faith was “counted to him as righteousness.” But the words “it was counted to him” were not written for his sake alone, but for ours also. It will be counted to us who believe in him who raised from the dead Jesus our Lord, who was delivered up for our trespasses and raised for our justification.