A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
lev 18:2UNLAWFUL MARRIAGES. (Lev. 18:1-30)
I am the Lord your God--This renewed mention of the divine sovereignty over the Israelites was intended to bear particularly on some laws that were widely different from the social customs that obtained both in Egypt and Canaan; for the enormities, which the laws enumerated in this chapter were intended to put down, were freely practised or publicly sanctioned in both of those countries; and, indeed, the extermination of the ancient Canaanites is described as owing to the abominations with which they had polluted the land.
lev 18:5Ye 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 (Luk 10:28) and the apostle (Rom 10:2).
lev 18:6None of you shall approach to any that is near of kin to him--Very great laxity prevailed amongst the Egyptians in their sentiments and practice about the conjugal relation, as they not only openly sanctioned marriages between brothers and sisters, but even between parents and children. Such incestuous alliances Moses wisely prohibited, and his laws form the basis upon which the marriage regulations of this and other Christian nations are chiefly founded. This verse contains a general summary of all the particular prohibitions; and the forbidden intercourse is pointed out by the phrase, "to approach to." In the specified prohibitions that follow, all of which are included in this general summary, the prohibited familiarity is indicated by the phrases, to "uncover the nakedness" [Lev 18:12-17], to "take" [Lev 18:17-18], and to "lie with" [Lev 18:22-23]. The phrase in this sixth verse, therefore, has the same identical meaning with each of the other three, and the marriages in reference to which it is used are those of consanguinity or too close affinity, amounting to incestuous connections.
lev 18:18Neither shalt thou take a wife to her sister, to vex her--The original is rendered in the Margin, "neither shalt thou take one wife to another to vex her," and two different and opposite interpretations have been put upon this passage. The marginal construction involves an express prohibition of polygamy; and, indeed, there can be no doubt that the practice of having more wives than one is directly contrary to the divine will. It was prohibited by the original law of marriage, and no evidence of its lawfulness under the Levitical code can be discovered, although Moses--from "the hardness of their hearts" [Mat 19:8; Mar 10:5] --tolerated it in the people of a rude and early age. The second interpretation forms the ground upon which the "vexed question" has been raised in our times respecting the lawfulness of marriage with a deceased wife's sister. Whatever arguments may be used to prove the unlawfulness or inexpediency of such a matrimonial relation, the passage under consideration cannot, on a sound basis of criticism, be enlisted in the service; for the crimes with which it is here associated warrant the conclusion that it points not to marriage with a deceased wife's sister, but with a sister in the wife's lifetime, a practice common among the ancient Egyptians, Chaldeans, and others.
lev 18:21thou shalt not let any of thy seed pass through the fire to Molech, &c.--Molech, or Moloch, which signifies "king," was the idol of the Ammonites. His statue was of brass, and rested on a pedestal or throne of the same metal. His head, resembling that of a calf, was adorned with a crown, and his arms were extended in the attitude of embracing those who approached him. His devotees dedicated their children to him; and when this was to be done, they heated the statue to a high pitch of intensity by a fire within, and then the infants were either shaken over the flames, or passed through the ignited arms, by way of lustration to ensure the favor of the pretended deity. The fire-worshippers asserted that all children who did not undergo this purifying process would die in infancy; and the influence of this Zabian superstition was still so extensively prevalent in the days of Moses, that the divine lawgiver judged it necessary to prohibit it by an express statute.
neither shalt thou profane the name of thy God--by giving it to false or pretended divinities; or, perhaps, from this precept standing in close connection with the worship of Molech, the meaning rather is, Do not, by devoting your children to him, give foreigners occasion to blaspheme the name of your God as a cruel and sanguinary deity, who demands the sacrifice of human victims, and who encourages cruelty in his votaries.
lev 18:24Defile not yourselves in any of these things--In the preceding verses seventeen express cases of incest are enumerated; comprehending eleven of affinity [Lev 18:7-16], and six of consanguinity [Lev 18:17-20], together with some criminal enormities of an aggravated and unnatural character. In such prohibitions it was necessary for the instruction of a people low in the scale of moral perception, that the enumeration should be very specific as well as minute; and then, on completing it, the divine lawgiver announces his own views of these crimes, without any exception or modification, in the remarkable terms employed in this verse.
in all these the nations are defiled which I cast out before you, &c.--Ancient history gives many appalling proofs that the enormous vices described in this chapter were very prevalent, nay, were regularly practised from religious motives in the temples of Egypt and the groves of Canaan; and it was these gigantic social disorders that occasioned the expulsion, of which the Israelites were, in the hands of a righteous and retributive Providence, the appointed instruments (Gen 15:16). The strongly figurative language of "the land itself vomiting out her inhabitants" [Lev 18:25], shows the hopeless depth of their moral corruption.
lev 18:25therefore I do visit the iniquity thereof upon it; and the land itself vomiteth out her inhabitants--The Canaanites, as enormous and incorrigible sinners, were to be exterminated; and this extermination was manifestly a judicial punishment inflicted by a ruler whose laws had been grossly and perseveringly outraged. But before a law can be disobeyed, it must have been previously in existence; and hence a law, prohibiting all the horrid crimes enumerated above--a law obligatory upon the Canaanites as well as other nations--was already known and in force before the Levitical law of incest was promulgated. Some general Iaw, then, prohibiting these crimes must have been published to mankind at a very early period of the world's history; and that law must either have been the moral law, originally written on the human heart, or a law on the institution of marriage revealed to Adam and known to the Canaanites and others by tradition or otherwise.
lev 18:29the souls that commit them shall be cut off--This strong denunciatory language is applied to all the crimes specified in the chapter without distinction: to incest as truly as to bestiality, and to the eleven cases of affinity [Lev 18:7-16], as fully as to the six of consanguinity [Lev 18:17-20]. Death is the punishment sternly denounced against all of them. No language could be more explicit or universal; none could more strongly indicate intense loathing and abhorrence.
lev 18:30Therefore shall ye keep mine ordinance, that ye commit not any one of these abominable customs--In giving the Israelites these particular institutions, God was only re-delivering the law imprinted on the natural heart of man; for there is every reason to believe that the incestuous alliances and unnatural crimes prohibited in this chapter were forbidden to all men by a law expressed or understood from the beginning of the world, or at least from the era of the flood, since God threatens to condemn and punish, in a manner so sternly severe, these atrocities in the practice of the Canaanites and their neighbors, who were not subject to the laws of the Hebrew nation.