Speak unto the children of Israel, and say unto them, I am the LORD your God.

Wesley's Notes

18:2 Your God - Your sovereign, and lawgiver. This is often repeated because the things here forbidden were practised and allowed by the gentiles, to whose custom he opposes divine authority and their obligation to obey his commands.

Jamieson-Fausset-Brown Bible Commentary

2-4. 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.

Matthew Henry's Concise Commentary

18:1-30 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.