John Gill Commentary Jeremiah 31:36

"If these ordinances depart from before me, saith Jehovah, then the seed of Israel also shall cease from being a nation before me for ever." — Jeremiah 31:36 (ASV)

If those ordinances depart from before me, saith the Lord ,
&c.] Of the sun, moon, and stars; should these leave their proper course, and not perform their several functions, or do the service appointed for them; should they desert their master, or disobey his orders, turn away from him, and pay no regard to the laws and rules he has set them: [then] the seed of Israel shall also cease from being a nation before
me for ever ;
but, as the former is impossible, so is the latter. The Jews ceased not from being a nation through their captivity in Babylon, nor through their destruction by the Romans; they continue a distinct nation and people to this day, though scattered throughout the nations of the world.

Though this rather refers to the spiritual Israel, the holy nation and peculiar people; Christ will have a seed to serve him as long as the sun and moon endure; his church shall continue to the end of the world; it is built on a rock; and the gates of hell cannot prevail against it.