John Gill Commentary 2 Chronicles 26:16

"But when he was strong, his heart was lifted up, so that he did corruptly, and he trespassed against Jehovah his God; for he went into the temple of Jehovah to burn incense upon the altar of incense." — 2 Chronicles 26:16 (ASV)

When he was strong
Became great and powerful, through his victories, the enlargement of his dominions, and having a numerous army, and these well accoutred, and many fortified cities and towers:

his heart was lifted up to his destruction ;
he grew vain and proud, elated with his flourishing circumstances, which issued in his ruin:

for he transgressed against the Lord his God ;
who had helped him, and made him so great, and had bestowed so many favours upon him; the Targum is, ``the Word of the Lord his God;'' what his transgression was, follows:

and went into the temple of the Lord ;
the holy place, into which none but the priests might enter:

to burn incense upon the altar of incense ;
which stood there.