It may not seem like a big deal to us, but in that time, it was not a king’s role to enter into the temple. This was a duty assigned to the priests. In other religions, especially in eastern culture, it was expected that the most significant person in the community (usually the king) would also be the one who would represent the people before their god. With God’s people, going back to Aaron (the first high priest), it was never the duty of a king.
This wasn’t the root issue, though. It was pride.
“But when he became strong, his heart was so proud that he acted corruptly, and he was unfaithful to the LORD his God, for he entered the temple of the LORD to burn incense on the altar of incense.”
2 Chronicles 26:16 NASB
When he became strong, his heart became proud. This says a lot. How many times have we looked at what others have and wished that we were them? Often, it is the bigger and better that peaks our interest. Here, we have a story of a man who became strong by his works but suffered terrible consequences spiritually. It stands as a warning that we cannot sacrifice spiritual growth for worldly gain.