Adam Clarke on Luke 1:54

He hath holpen [supported, αντελαβετο] his servant Israel - Israel is here represented as falling, and the Lord comes speedily in and props him up. The house of David was now ready to fall and rise no more; Jesus, being born of the very last branch of the regal line, revived the family, and restored the dominion.

In remembrance of his mercy - By mercy, the covenant which God made with Abraham, Gen 15:18, is intended; which covenant proceeded from God's eternal mercy, as in it salvation was promised to all the nations of the earth. See Gen 17:19, and Gen 22:18, and this promise was, in one form or other, given to all the fathers, Luk 1:55.

This song properly consists of three parts.

1. In the first part Mary praises God for what he had done for herself, Luk 1:46-50.

2. In the second, she praises him for what he had done, and would do, against the oppressors of his people, Luk 1:51-53.

3. In the third, she praises him for what he had done, and would do, for his Church, Luk 1:53-56.