He wasn't fully committed to serving God. He showed up for his religious ceremonies, but then took revenge for his father's death into his own hands (2 Chronicles 25:3). He listened to the man of God sometimes, as is shown in 2 Chronicle 25:7-10. But then on his return from that same battle, he brought back their idles and started worshiping them (2 Chronicles 25:14-15).
God won't honor a partial commitment to serve Him. Matthew 6:24 says you can't serve two masters. If there's sin in your life and you're covering it up and you won't commit that to God, you're serving your sin. If you don't have time for God any time of the day or night then you're not fully serving God.
We see in 2 Chronicles 25:27-28 that from the time he turned away following God, his people began to conspire against him so much that he ended up running for his life and being hunted down and killed. While you aren't likely today to be hunted and killed for your rejection of God, when those who look to you for an example see the sin that you harbor in your life, there will be disappointment, resentment, and anger just the same.
Turn your sin over to God and follow Him with your whole heart, so that your testimony can be better than that of Amaziah. If you turn your heart completely over to God, you can have the testimony, "He did what was right in the eyes of the Lord wholeheartedly." It will make a difference in your life that you will never regret.
