(For the LORD thy God is a merciful God;) he will not forsake thee, neither destroy thee, nor forget the covenant of thy fathers which he {u} sware unto them.

(u) To certify to them even more the assurance of their salvation.

Matthew Henry's Concise Commentary

4:24-40 Moses urged the greatness, glory, and goodness of God. Did we consider what a God he is with whom we have to do, we should surely make conscience of our duty to him, and not dare to sin against him. Shall we forsake a merciful God, who will never forsake us, if we are faithful unto him? Whither can we go? Let us be held to our duty by the bonds of love, and prevailed with by the mercies of God to cleave to him. Moses urged God's authority over them, and their obligations to him. In keeping God's commandments they would act wisely for themselves. The fear of the Lord, that is wisdom. Those who enjoy the benefit of Divine light and laws, ought to support their character for wisdom and honour, that God may be glorified thereby. Those who call upon God, shall certainly find him within call, ready to give an answer of peace to every prayer of faith. All these statutes and judgments of the Divine law are just and righteous, above the statutes and judgments of any of the nations. What they saw at mount Sinai, gave an earnest of the day of judgment, in which the Lord Jesus shall be revealed in flaming fire. They must also remember what they heard at mount Sinai. God manifests himself in the works of the creation, without speech or language, yet their voice is heard, Ps 19:1,3; but to Israel he made himself known by speech and language, condescending to their weakness. The rise of this nation was quite different from the origin of all other nations. See the reasons of free grace; we are not beloved for our own sakes, but for Christ's sake. Moses urged the certain benefit and advantage of obedience. This argument he had begun with, ver. 1, That ye may live, and go in and possess the land; and this he concludes with, ver. 40, That it may go well with thee, and with thy children after thee. He reminds them that their prosperity would depend upon their piety. Apostacy from God would undoubtedly be the ruin of their nation. He foresees their revolt from God to idols. Those, and those only, shall find God to their comfort, who seek him with all their heart. Afflictions engage and quicken us to seek God; and, by the grace of God working with them, many are thus brought back to their right mind. When these things are come upon thee, turn to the Lord thy God, for thou seest what comes of turning from him. Let all the arguments be laid together, and then say, if religion has not reason on its side. None cast off the government of their God, but those who first abandon the understanding of a man.