Charles Ellicott Commentary 2 Kings 21:3

"For he built again the high places which Hezekiah his father had destroyed; and he reared up altars for Baal, and made an Asherah, as did Ahab king of Israel, and worshipped all the host of heaven, and served them." — 2 Kings 21:3 (ASV)

For he built up again. —The Septuagint and Vulgate imitate the Hebrew idiom, and he returned and built—i.e., and he rebuilt.

The high places ... altars for Baal ... a grove (an Ashçrah). —“The idols, the sun-pillars, the ashçrim, the sacred trees, and all the other pagan or half-pagan symbols, so plainly inconsistent with the prophetic faith, were of the very substance of Israel’s worship in the popular sanctuaries” (Prof. Robertson Smith).

As did Ahab. —See 1 Kings 16:32–33.

Worshipped all the host of heaven. —See Notes on 2 Kings 17:16, and compare 2 Kings 23:12. The Babylonian star-worship and astrology, with concomitant superstitions, had been introduced under Ahaz.