In verses 51-53 we read:
“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 empty away. “I think Mary is showing her vast knowledge of the scriptures here . . . she is reminding herself (and us) of the many times that God has overturned the things of this world, and shown Himself to be merciful and fair.
Why would that have been important to her? Think about the time in which she and Joseph and all the others were living . . . they lived under the rule of Rome. What a system! Rome's society was built on (and glorified) an elite, racist, oppressive world-view. It ensured that the rich would remain rich (or get more wealthy) and the poor would remain poor (or get even more destitute). The system took advantage of the worker, the peasant, the farmer, and handed that advantage to the elite. This system walked all over the handicapped, the sick, the widows, and the orphans.
And to add insult to injury, the Romans were not the only ones. The wealthy leaders of the Jewish people loved their power, too, and would tread upon anyone who wanted to change things. They had rules and regulations, and then rules about the rules; they bound everyone up in rituals and there seemed to be no way to change things.
But Mary is realizing, and speaks in her psalm, about the "new program." God is going to change things! No longer will it be the arrogant, the mighty, and the rich who win. God is a God of justice, and He is aware of those who are proud in their inmost thoughts. Those who are arrogant and believe that only they are good enough -- they've made it!
But Mary sees that their fortunes will be overturned. The ones that think they are so wonderful that they are legendary are going to be the ones whose lives will be turned upside down!
God can turn human attitudes and the rules of society upside down -- He acts with awesome power and scatters the prideful. He is going to disperse them. Dethrone them. Deprive them. Just as they sought to deprive others of things they needed.
Then, the needs of the poor, whether they are spiritual or physical, will be met (abundantly) by Him.
This Son that she would bear would say: