Neither before nor after Josiah was there a king like him who turned to the Lord as he did--with all his heart and with all his soul and with all his strength, in accordance with the Law of Moses.
Now if you go back and read chapters 22 and 23 leading up to this, you see the simple pattern:
1. He heard the law--His mind.
2. He committed himself to the law.--His Spirit/soul
3. Finally, he obeyed the law--His strength.
I know a lot of folks like to claim we are no longer under the law, but Jesus Himself said to love the Lord your God with all your heart and all your mind and all your soul and all your strength. This is the greatest commandment. When asked how one should live, THIS is what Jesus said.
Notice Josiah was not commended for believing there is a God. He was commended for how he chose to serve Him.
Interesting that Jesus gave his service and commitment another name--Love.