Theophylact of Ohrid

"He has shown mercy" not only to the living, but also "to our fathers," for the grace of Christ extended to them as well, even though they had already died. Namely: to us the living He granted the hope of resurrection, and we shall rise; but not we alone shall be deemed worthy of this benefaction, but also those who died before us. For all of nature received this blessing. And in another sense: "He showed mercy to the fathers" in that He fulfilled their expectations, for what they had hoped for, they saw fulfilled in Christ. …

Commentary on Luke ↗

Bede

Having announced that the Lord, according to the declaration of the Prophet, would be born of the house of David, he now says, that the same Lord to fulfil the covenant He made with Abraham will deliver us, because chiefly to these patriarchs of Abraham's seed was promised the gathering of the Gentiles, or the incarnation of Christ. But David is put first, because to Abraham was promised the holy assembly of the Church; whereas to David it was told that from him Christ was to be born. …

Catena Aurea by Aquinas ↗

Bonaventure

To perform mercy, etc. This is the third principal part of the canticle, in which he gives thanks for our restoration, inasmuch as it was promised to the Fathers under oath. And in this part he first sets forth the principle moving to the promise; second, the oath establishing it; and third, the remedy following upon it. The principle now moving to the promise was the divine mercy, from which promise arose the testimony of the Fathers: and he touches on this when he says: To perform mercy with our Fathers. …

Commentary on Luke, Chapter 1 ↗