Cyril of Alexandria

For the law was not a perpetual and immoveable law, but it remained in force only as long as the time of its imposition. As the most wise Paul said, "For the former and ancient command has a gap whose filling is to be sought in the second, that is, the new has many things quite different from the old." …That second one is the eternal covenant, and that he fulfilled it for those approaching him by faith is confirmed when it immediately adds, "the holy trustworthy things of David." This concerns the announcements about the Savior of us …

Commentary On Isaiah 5:2.55:1-5 ↗

Jerome

The promise of all good things is eternal life. But if you wish to hear and your soul will live forever, I will make with you an everlasting covenant, the faithful mercies of David.… But this pact, which the Lord promises, will not be brief and of one time, as it was for the people of the Jews, but it will remain forever, so that the true David may come, and what has been promised from the person of God may be fulfilled in the Gospel: "I have found David my servant, with my holy oil I have anointed him." … Whom, according to Ezekiel, …

Commentary on Isaiah ↗

Thomas Aquinas

Second, he promises escape from death: incline your ear: my soul shall live (Ps 118:175); and also: your justifications I will never forget: for by them you have given me life (Ps 118:93). Third, he promises power. And first, he sets out a similitude: and I will make an everlasting covenant, the faithful mercies of David, that is, as I made true promises to David that his seed would remain forever, so I will also do with you.

Commentary on Isaiah ↗