30:20 That thou mayest love the Lord thy God - Here he shews them in short, what their duty is; To love God as the Lord, a being most amiable, and as their God, a God in covenant with them: as an evidence of their love, to obey his voice in every thing, and by constancy in this love and obedience, to cleave to him all their days. And what encouragement had they to do this? For he is thy life and the length of thy days - He gives life, preserves life, restores life, and prolongs it, by his power, tho' it be a frail life, and by his presence, tho' it be a forfeited life. He sweetens life by his comforts, and compleats all in life everlasting.
30:15-20 What could be said more moving, and more likely to make deep and lasting impressions? Every man wishes to obtain life and good, and to escape death and evil; he desires happiness, and dreads misery. So great is the compassion of the Lord, that he has favoured men, by his word, with such a knowledge of good and evil as will make them for ever happy, if it be not their own fault. Let us hear the sum of the whole matter. If they and theirs would love God, and serve him, they should live and be happy. If they or theirs should turn from God, desert his service, and worship other gods, that would certainly be their ruin. There never was, since the fall of man, more than one way to heaven; which is marked out in both Testaments, though not with equal clearness. Moses meant that same way of acceptance, which Paul more plainly described; and Paul's words mean the same obedience, on which Moses more fully treated. In both Testaments the good and right way is brought near, and plainly revealed to us.