Thomas Aquinas Commentary Jeremiah 33:19-22

"And the word of Jehovah came unto Jeremiah, saying, Thus saith Jehovah: If ye can break my covenant of the day, and my covenant of the night, so that there shall not be day and night in their season; then may also my covenant be broken with David my servant, that he shall not have a son to reign upon his throne; and with the Levites the priests, my ministers. As the host of heaven cannot be numbered, neither the sand of the sea measured; so will I multiply the seed of David my servant, and the Levites that minister unto me." — Jeremiah 33:19-22 (ASV)

Here, the confirmation of the promise is given:

1. From the duration of created things: if my covenant with the day can be broken; it is as if he were saying, “It cannot be broken until the end of the world.” I shall establish his seed forever, and his throne as the days of heaven (Psalms 88:30).
2. From their multitude: as the stars of heaven cannot be numbered, except by God alone; so the number of the elect is known only to God. I shall multiply your seed like the stars of heaven, and like the sand which is on the shore of the sea (Genesis 22:17).