And the Lord spoke unto Moses. I have not introduced this declaration amongst other similar ones, which had for their object the preparation of their minds for the reverent reception of the Law, because, whatever conformity there may be in the words themselves, in their substance there is a great difference; for they were general, whereas this is specially confined to a single point. For it was not God's intention here merely to exhort the people to the study of the Law, but the address respecting the keeping of His statutes is directed to the present cause, since He does not refer indifferently to all the statutes of Himself and of the Gentiles, but restricts Himself to the subject-matter, as it is called; and thus, by the statutes of the Gentiles, He means those corruptions whereby they had perverted His pure institution as to holy matrimony. First, however, tie forbids them from following the customs of the Egyptians, and then includes all the Canaanitish nations. For, since all the Orientals are libidinous, they never had any scruple in polluting themselves by incestuous marriages; whilst it is abundantly proved by history, how great were the excesses of the Egyptians [1] in this respect. A brother had no abhorrence against marrying his uterine sister, nor a paternal or maternal uncle his niece; in a word, they were so dead to. shame that they were carried away by their lusts to trample upon all the laws of nature. This is the reason why God here enumerates the kinds of incest of which the mention would else have been superfluous.

Footnotes

1 - "A very objectionable custom, which is not only noticed by Diodorus, but is fully authenticated by the sculptures both of Upper and Lower Egypt, existed among them from the earliest times, the origin and policy of which it is not easy to explain -- the marriage of brother and sister, which Diodorus supposes to have been owing to, and sanctioned by, that of His and Osiris; but as this was purely an allegorical fable, and these ideal personages never lived on earth, his conjecture is of little weight; nor does any ancient writer offer a satisfactory explanation of so strange a custom." -- Wilkinson's Popular Account of the Ancient Egyptians, 2:224.

And the Lord spake unto Moses,.... He continued speaking to him, after he had delivered to him the laws respecting the day of atonement, and the bringing of the sacrifices to the door of the tabernacle, and particularly concerning the Israelites not worshipping devils, as they had done in Egypt: the Lord proceeds to deliver out others, the more effectually to guard against both the immoral and idolatrous practice, of the Egyptians and Canaanites:
saying, as follows.

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.

Holiness of the Marriage Relation. - The prohibition of incest and similar sensual abominations is introduced with a general warning as to the licentious customs of the Egyptians and Canaanites, and an exhortation to walk in the judgments and ordinances of Jehovah (Leviticus 18:2-5), and is brought to a close with a threatening allusion to the consequences of all such defilements (Leviticus 18:24-30).