| Barnes' Notes on the Bible If a man keeps the "statutes" (i. e. the ordinances of Leviticus 18:4) and "judgments" of the divine law, he shall not be "cut off from his people" (compare Leviticus 18:29), he shall gain true life, the life which connects him with Yahweh through his obedience. See the margin reference and Luke 10:28; Romans 10:5; Galatians 3:12.
Gill's Exposition of the Entire BibleYe shall therefore keep my statutes, and my judgments,.... The same as before; these they were to keep in their minds and memories, and to observe them and do them: which if a man do he shall live in them; live a long life in the land of Canaan, in great happiness and prosperity, see Deuteronomy 30:20; for as for eternal life, that was never intended to be had, nor was it possible it could be had and enjoyed by obedience to the law, which fallen man is unable to keep; but is what was graciously promised and provided the covenant of grace, before the world was, to come through Christ, as a free gift to all that believe in him, see Galatians 3:11; though some Jewish writers interpret this of eternal life, as Jarchi, Aben Ezra, and Ben Gersom: I am the Lord; that has enjoined these statutes and judgments, and promised life to the doers of them, able and faithful to perform what is promised.
Keil and Delitzsch Biblical Commentary on the Old Testament"The man who does them (the ordinances of Jehovah) shall live (gain true life) through them" (see at Exodus 1:16 and Genesis 3:22).
Geneva Study BibleYe shall therefore keep my statutes, and my judgments: which if a man do, he shall live in them: {b} I am the LORD. (b) And therefore you ought to serve me alone, as my people.
Wesley's Notes 18:5 He shall live in them - Not only happily here, but eternally hereafter. This is added as a powerful argument why they should follow God's commands, rather than mens examples, because their life and happiness depend upon it. And though in strictness, and according to the covenant of works they could not challenge life for so doing, except their obedience was universal, perfect, constant and perpetual, and therefore no man since the fall could be justified by the law, yet by the covenant of grace this life is promised to all that obey God's commands sincerely.
Jamieson-Fausset-Brown Bible Commentary5. Ye shall therefore keep my statutes, and my judgments: which if a man do, he shall live in them-A special blessing was promised to the Israelites on condition of their obedience to the divine law; and this promise was remarkably verified at particular eras of their history, when pure and undefiled religion prevailed among them, in the public prosperity and domestic happiness enjoyed by them as a people. Obedience to the divine law always, indeed, ensures temporal advantages; and this, doubtless, was the primary meaning of the words, "which if a man do, he shall live in them." But that they had a higher reference to spiritual life is evident from the application made of them by our Lord (Lu 10:28) and the apostle (Ro 10:2).
Matthew Henry's Concise Commentary18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services. | 
