Albert Barnes Commentary 2 Kings 16:3

"But he walked in the way of the kings of Israel, yea, and made his son to pass through the fire, according to the abominations of the nations, whom Jehovah cast out from before the children of Israel." — 2 Kings 16:3 (ASV)

Ahaz was the worst of all the kings of Judah. He imitated the worst of the Israelite kings—Ahab and Ahaziah—by re-introducing the worship of Baal, which had been rooted out of Israel by Jehu and out of Judah by Jehoiada.

And made his son to pass through the fire—that is, Ahaz adopted the Moloch worship of the Ammonites and Moabites (2 Kings 3:27; Micah 6:7). He sacrificed at least one son, probably his firstborn, according to the horrid rites of those nations and the Canaanite tribes (Deuteronomy 12:31; Psalms 106:37–38).

Before this time, it seems, the Jews had been guiltless of this abomination. They had been warned against it by Moses (Deuteronomy 18:10). And if, as some think, they had practiced it in the wilderness (Ezekiel 20:26; Amos 5:26), the sin must have been rare and exceptional, for from the time they entered the promised land, they had completely put it away.

Now, however, this sin became so frequent (compare 2 Kings 17:17; 21:6) that it was met with the strongest protests from Jeremiah and Ezekiel (Jeremiah 7:31–32; Jeremiah 19:2–6; Jeremiah 32:35; Ezekiel 16:20; Ezekiel 20:26; Ezekiel 23:37).