I finally figured out what book it was.
I did not have all the details right, but leaving this answer for anyone else that stumbles upon this thread.
I was referring to the Noble Warriors trilogy.
The Noble Warriors trilogy is a fantasy series, written by British novelist William Nicholson. The first book, Seeker, was published in 2006, as was the second in the trilogy, Jango. The third book, Noman, was published in September 2007.
A man named the Brother once resided on the island of Anacrea, and one night while he was sleeping, a lost child came to him, asking him for help. He let the Lost Child in, and later that night, in a dream, the Lost Child spoke to the Brother, proclaiming himself to be the 'All and Only', the 'Reason and the Goal.' Then the child changed form and became a woman, who called herself the Loving Mother. The figure changed form twice more, naming itself the Wounded Warrior and the Wise Father. The Wise Father spoke of an Assassin who would eventually kill him.
Following the dream, the Brother built a garden for the Lost Child to live in, believing him to be the one true god. Others came to the island and they too pledged to protect the Lost Child.
One day, a powerful warlord named Noman came to the island. He forced his way into the garden to see the child. He stayed there for a day and a night and when he came out, he never spoke of what he saw. Soon after, he disbanded his army and formed a community to protect the god, which became known as the Nom, of which he was the leader. Noman learned the secret of true strength and he passed his knowledge to his brothers and sisters and they too became strong. Members of the Nom are called Nomana, or the Noble Warriors. The Noble Warriors do not use weapons or armour; they use only true strength, which is the life force in all beings called Lir
The vow of the noble warriors (which Noman wrote) says they cannot fight long wars or conquer land or empires. They cannot love any person above all others, build a lasting home, possess anything but can only use their powers to bring freedom to the enslaved and justice to the oppressed.
In Jango, it is mentioned that Noman lived on Anacrea over 200 years ago. Nothing has been said of his death but one of Seeker's teachers said that Noman left them to "submit himself to his final test. He was never seen again".