18:30-32 The Lord will judge each of the Israelites according to his ways. On this is grounded an exhortation to repent, and to make them a new heart and a new spirit. God does not command what cannot be done, but admonishes us to do what is in our power, and to pray for what is not. Ordinances and means are appointed, directions and promises are given, that those who desire this change may seek it from God.

31. Cast away from you—for the cause of your evil rests with yourselves; your sole way of escape is to be reconciled to God (Eph 4:22, 23).

make you a new heart—This shows, not what men can do, but what they ought to do: what God requires of us. God alone can make us a new heart (Eze 11:19; 36:26, 27). The command to do what men cannot themselves do is designed to drive them (instead of laying the blame, as the Jews did, elsewhere rather than on themselves) to feel their own helplessness, and to seek God's Holy Spirit (Ps 51:11, 12). Thus the outward exhortation is, as it were, the organ or instrument which God uses for conferring grace. So we may say with Augustine, "Give what thou requirest, and (then) require what thou wilt." Our strength (which is weakness in itself) shall suffice for whatever He exacts, if only He gives the supply [Calvin].

spirit—the understanding: as the "heart" means the will and affections. The root must be changed before the fruit can be good.

why will ye die—bring on your own selves your ruin. God's decrees are secret to us; it is enough for us that He invites all, and will reject none that seek Him.