Amaziah didn't have it all together though. 2nd Chronicles 25:2 tells us that his heart was not perfect. He was a bit of a pragmatist in the exercise of his faith. He did what was right... sometimes, but in some of those cases he evidently did right only because he could see that it was in his immediate best interest; he felt like he was going to get something out of it. That's a dilemma all of us should consider. It is one thing to do what is right when there are benefits. It's quite another thing altogether to do what's rights when there are negative consequences.
God made good on His offer to assist Amaziah in war. However, Amaziah didn't act very loyally after his success. Probably in a reaction to the maltreatment which his people endured from the dismissed Jewish mercenaries, Amaziah brought back idols from the Edomites and set them up in Judah. Question: if the Edomite deities weren't strong enough to deliver the Edomites from the Judean attack, why did the Judeans want to take those gods for themselves (2nd Chronicles 25:15)?
As you might expect, things went downhill from there for King Amaziah. He ended up picking a fight with the same army that he had previously hired to help his own army. And, he was later assassinated.