Albert Barnes 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 - If these. From the uniformity of God’s operations in the material world, the prophet deduces the certainty of a similar uniformity in God’s dealings with man in things spiritual.

A nation - A people. Israel has long ceased to be a nation, but it still exists as a numerous, influential, and distinct people. In (Matthew 28:19–20) Jeremiah’s prophecy receives its Christian application, and Israel becomes the Church, with the promise of perpetual existence. It has no national existence, but its members ought to be a strongly marked people, refusing to be merged in the world, while everywhere they pervade and influence it.