Lévitique 18:21

And of thy offspring thou shalt not give up any to lie carnally with the daughters of the Gentiles, to perform strange worship; nor shalt thou profane the Name of thy God: I am the Lord.

Molekh the name of an idol. Our Sages, of blessed memory, have interpreted this word [Sanhedrin 64a] as a general term for anything that someone may set on a throne [Hebrew: yamlikh] to rule over him. In all likelihood, however, it is also the name of a specific Ammonite abomination [I Kings 11:7].

to hand over meaning, to burn — for this was the manner of its worship. Others say that the child was passed over a fire: some survived, and others died. Still others point out that the verse nowhere mentions fire, and that the commandment is not to hand a child over from the teachings of God into the worship of Molekh. By handing over the offspring of the holy nation to Molekh, you thereby desecrate the Name of your God, Who has called you into His service. Sa‘adya Gaon classified the sexual crimes into eight levels of vileness [comment on Exodus 20:13]. Worst of all is intercourse with a beast, because that involves an entirely different species. The next level of severity is intercourse with another man, because the entire gender is forbidden. A lower level of severity is intercourse with one’s father’s wife, and similar relations who are always forbidden. I shall not belabor the point, because these classifications are not practically useful, insofar as all sexual offenses are punishable by karet .

AND THOU SHALT NOT GIVE ANY OF THY SEED ‘L’HA’AVIR’ (TO PASS THROUGH) ‘LAMOLECH’ (TO MOLECH). This was an idol the name of which was “Molech.” Scripture mentions it with the definite article, [in the word lamolech] is vowelled with a pathach [instead of with a shva which would have made it l’molech], it is quite clear that it is speaking of an idol that is known. See Ramban on Exodus 22:19. (Vol. II, p. 390) quoting Rashi. since it was well-known to them from Egypt. And Rabbi Abraham ibn Ezra wrote that it is possible that Molech is identical with Milcom the detestation of the Ammonites. And so indeed it would appear, for it is written, Then did Solomon build a high place for Chemosh the detestation of Moab … and for Molech the detestation of the children of Ammon., Verse 7. Milcom of Verse 5, is thus identical with Molech of Verse 7. Hence the idol by the name of Molech mentioned here in the Torah appears to be, as Ibn Ezra wrote, Milcom the detestation of the Ammonites. And this [name Milcom] was also well-known to them., when from the text here before us in the Torah it is plain that they knew the idol already from Egypt [as is explained in the text above]? To this question Ramban answers that it was also known to them by the name “Milcom” [in the time of Solomon]. Hence Scripture used that name.
Now Rashi commented: “And this was the manner in which the Molech was worshipped: he [the father] would hand over his son to the priests, and the priests lit two large pyres [one opposite the other], and they made the child pass on foot between the two fires. Thou shalt not give, this refers to [the father] handing over [the child] to the priests. ‘L’ha’avir lamolech’ is the passing through the open fire.” But this is not correct. For the Rabbis have already said in the Gemara of Tractate Sanhedrin: “If he made him pass on foot [between the fires] the father is free from punishment [because that was not the customary way of worshipping the idol]; rather, [he is liable only if it was] ‘like a leaping place of Purim,’”). The Talmudic text k’mishvarto d’puria is thus explained by Rashi “as a leaping place of Purim,” i.e., like a bonfire made on Purim for merriment, over which children leap. Ramban will later on explain that at the Molech the child was passed through the flames so many times that the flames actually burnt him. meaning that he made him pass through the actual …

ומזרעך לא תתן להעביר למולך ולא תחלל את שם אלוקיך, the difference between offering sacrifices to the Moloch and to G’d respectively is that to G’d only animals are sacrificed, whereas to the deity known as Moloch, human beings, specifically one’s son, is sacrificed. This would indicate that the worshipper of the Moloch considers him as more powerful than G’d, for why else would he sacrifice his dearest possession, his son, to him and not to G’d?

'אני ה, I have never changed since the time I have sworn to Avraham “to be your G’d and for seed after you.” (Genesis 17,7). Seeing that G’d in this chapter had spoken about incest, matters related to human seed, and His interest in keeping this seed pure so that the holy spirit would be able to descend upon such people, He spoke about the Moloch whose servants had intended to ensure that their other children would be blessed with every kind of success in recognition of their having given the oldest to their G’d. G’d is on record here that doing something like this, far from being a way to sanctifying G’d’s name would be the most potent way to desecrating His name. Such practices, far from attracting the benevolent presence of G’d, the Shechinah, would prevent this presence of G’d from taking up residence among the Jewish people..