John Gill Commentary 2 Kings 16

In the seventeenth year of Pekah the son of Remaliah Ahaz
the son of Jotham king of Judah began to reign .
] Jotham began to reign in the second of Pekah, and he reigned sixteen years, and therefore his last year would fall in the eighteenth of Pekah; but as his first year might be at the beginning of the second of Pekah, his last was towards the end of the seventeenth of Pekah's, as here; see (2 Kings 15:32) .

Twenty years old was Ahaz when he began to reign, and reigned
sixteen years in Jerusalem
The same number of years his father did:

and did not that which was right in the sight of the Lord God, like
David his father ;
his more remote progenitor, nor even like his more immediate father, from whom he received such good instructions, and of whom he had so good an example; but grace is neither propagated by blood, nor obtained through the force of education.

But he walked in the way of the kings of Israel .
&c.] Worshipping the calves as they did; which, as it was contrary to the religious sentiments in which he was educated, so against his political interest, which was the only, or at least the principal thing, which swayed with the kings of Israel to continue that idolatry:

yea, and made his son to pass through the fire ;
between two fires to Molech, by way of lustration; which might be true of Hezekiah his son, and others of his sons, for he had more he burnt with fire, as appears from (2 Chronicles 28:3) , both ways were used in that sort of idolatry, (See Gill on Leviticus 18:21),

according to the abominations of the heathen, whom the Lord cast out
from before the children of Israel ;
the old Canaanites; so the Carthaginians, a colony of the Phoenicians, used in time of calamity to offer human sacrifices, and even their children, to appease their deities ^F12 . Theodoret says, he had seen in some cities, in his time, piles kindled once a year, over which not only boys, but men, would leap, and infants were carried by their mothers through the flames; which seemed to be an expiation or purgation, and which he takes to be the same with the sin of Ahaz.

FOOTNOTES:

• F12: Justin. e Trogo, Hist. l. 18. c. 6. Curt. Hist. l. 4. c. 3. Pescennius Festus apud Lactant. de fals. Relig. l. 1. c. 21.

And he sacrificed and burnt incense in the high places, and on
the hills
Which none of the kings of Judah before him ever did; for though they connived at this practice in the people, they never encouraged it by their own example; and very probably he offered sacrifices there to idols, see (2 Chronicles 28:25) whereas the people sacrificed to the true God, though at a wrong place:

and under every green tree ;
and which is never said of the people, and seems to confirm it, that Ahaz sacrificed to other gods, since the Heathens used to place idols under green trees, and worship them, whom the Jews imitated, (Jeremiah 2:2) (3:6) .

Then Rezin king of Syria, and Pekah son of Remaliah king of Israel, came up to Jerusalem to war To fight with Ahaz, moved to it by the Lord, to chastise Ahaz for his idolatry, (2 Kings 15:37) (2 Chronicles 28:5).

but could not overcome him so as to take Jerusalem, and set up another king there, as their scheme was, (Isaiah 7:5) though they had both at other times got great advantages over him, and slew many of his people, and carried them captive, see (2 Chronicles 28:5–8).