Albert Barnes Commentary Leviticus 20:2-5

"Moreover, thou shalt say to the children of Israel, Whosoever he be of the children of Israel, or of the strangers that sojourn in Israel, that giveth of his seed unto Molech; he shall surely be put to death: the people of the land shall stone him with stones. I also will set my face against that man, and will cut him off from among his people; because he hath given of his seed unto Molech, to defile my sanctuary, and to profane my holy name. And if the people at the land do at all hide their eyes from that man, when he giveth of his seed unto Molech, and put him not to death; then I will set my face against that man, and against his family, and will cut him off, and all that play the harlot after him, to play the harlot with Molech, from among their people." — Leviticus 20:2-5 (ASV)

Molech, literally, “the King”, also called Moloch, Milcom, and Malcham, was known in later times as “the abomination of the Ammonites” (1 Kings 11:5). He appears to have been the fire-god of the eastern nations, related to, and sometimes made identical with, Baal, the sun-god. The nature of the rite and of the impious custom called passing children through the fire to Molech is very doubtful. These practices appear to have been essentially connected with magical arts, probably also with unlawful lusts, and with some particular form of profane swearing. The rite in the time of Moses belonged to the region of magic rather than definite idolatrous worship, and may have been practiced as a lustral charm, or fire-baptism, for the children of incest and adultery.

Stone him with stones (Leviticus 20:2) – This was the commonest form of capital punishment. It was probably preferred as being the one in which the execution was the act of the whole congregation.

Defile my sanctuary (Leviticus 20:3) – that is, to pollute the people as identified with their sanctuary.