John Calvin Commentary Deuteronomy 4:30

"When thou art in tribulation, and all these things are come upon thee, in the latter days thou shalt return to Jehovah thy God, and hearken unto his voice:" — Deuteronomy 4:30 (ASV)

When thou art in tribulation. He here shows the advantage of punishments, on the basis of their usefulness and benefit; for what the Apostle says is confirmed by experience:

no chastening for the present seemeth to be joyous, but grievous; nevertheless afterward it yieldeth the peaceable fruit of righteousness to them that are exercised thereby. (Hebrews 12:11).

Therefore, lest they should be provoked to wrath by God’s stripes, he reminds them of their usefulness, because they would never turn to God unless aided by this remedy.

He tells them that, after they have been afflicted by the curses of God, if they sought Him, they would find Him. Furthermore, he gives them grounds for hope both in God’s nature and in His covenant.

He assures them that God will be willing to be appeased, because He is by nature merciful; but he adds another confirmation of this, which is more certain and familiar, namely, because God had adopted them by a perpetual covenant.