Josiah is viewed as a righteous King who sought to do what was right in the eyes of God (2 Chronicles 34). Yet it took time for his faith to evolve to a point where he acted on that faith. While he became King when he was eight, scripture tells us he began to seek God when he was 16. Then when he was twenty, he began to purge the kingdom of all the idol worship. It took him 12 years to get to that point.
Growing in our faith in Jesus is a journey that deepens our faith and trust in God. It starts with a belief that Jesus is Lord and a commitment to follow Him. Over time, as you pursue a deeper relationship with Christ, the Spirit will point out things in your life that you need to get rid of. As you align your life with God, your faith and desire to serve Him will grow stronger.
(NASB) 3 For in the eighth year of his reign while he was still a youth, he began to seek the God of his father David; and in the twelfth year he began to purge Judah and Jerusalem of the high places, the Asherim, the carved images and the molten images. 2 Chronicles 34:3, NASB
For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. – John 3:16, NASB