John Gill Commentary 2 Kings 21

Manasseh was twelve years old when he began to reign
So that he was born three years after Hezekiah's recovery from his sickness, and in the seventeenth year of his reign:

and reigned fifty five years in Jerusalem :
among which must be reckoned the time of his captivity in Babylon; his reign was the longest of any of the kings of Judah: and his mother's name was Hephzibah; the name the church goes by, and signifies, "my delight or pleasure is in her", (Isaiah 62:4) , no doubt she was a good woman, or Hezekiah would not have made choice of her for a wife; it is a tradition of the Jews ^F1 , that she was the daughter of Isaiah, whose name, they say, is not mentioned, because so wicked a king was unworthy of such a grandfather.

FOOTNOTES:

• F1: Hieron. Trad. Heb. in lib. Paralipom. fol. 86. F.

And he did that which was evil in the sight of the Lord
Was guilty of idolatry:

after the abomination of the Heathen, whom the Lord cast out
before the children of Israel:
the old Canaanites; he committed idolatry in imitation of them, and as the Phoenicians now did before the children of Israel.

For he built up again the high places which Hezekiah his
father had destroyed
The temples and altars upon them, see (2 Kings 18:4),

and he reared up altars for Baal ;
in the high places he rebuilt:

and made a grove, as did Ahab king of Israel :
which was either an idol itself, or a shade of trees where idols were placed; or rather Asherah, rendered "agrove", is the same with Astarte, the goddess of the Zidonians, the figure of which he made and worshipped;for groves were not so soon and easily planted, raised, and made; so the same in (1 Kings 16:33)

and worshipped all the host of heaven, and served them :
the sun, moon, and stars, particularly the planets Saturn, Jupiter, Mars, Mercury, and Venus.

And he built altars in the house of the Lord
In the holy place, as distinct from the courts in the next verse; and these were sacred to the idols of the Gentiles:

of which the Lord said, in Jerusalem will I put my name ;
in the temple there, devoted to his service, called by his name, and where his name was called upon, see (Deuteronomy 12:5) (16:6) and to erect altars to idols here must be very abominable to him.

And he built altars for all the host of heaven
Sun, moon, and stars:

in the two courts of the house of the Lord ;
in the court of the priests, and in the court of the people; and all this must be supposed to be done, not as soon as he began to reign, but when he was grown up to man's estate, and had children, as the next verse shows; unless it can be thought that those nobles in Judah, who liked not the reformation made by Hezekiah, took the advantage of his youth, and advised him to these idolatries.