This depends on the Era. Thankfully this lies entirely within my specialty! Such a rarity.

To answer your question it depends on the Era. During my specialty (War of the Hundred Years until the First Whitecloak Wars after the hanging of the body of Serenia Latar) the Whitecloaks were founded and warped.

Initially with the writings of Lothair Mantelar during the War of the Hundred years, he founded the Children of the Light. He feared the collapse of Hawkwing's reign was bringing too much Darkness into the hearts of the Westlands, allowing Darkfriends to rise to Power. As the first Lord Captain Commander, he had a unique place in the War of the Hundred Years. He fought in the ever turbulent landscape of the time, but not as most other generals did. Instead of trying to seize land, he was truely trying to save souls in a misguided attempt to cleanse the land. He said the temptation to wield the power could corrupt, and a special eye on those who could channel was warranted.

However once the War of the Hundred Years was over and the boundaries had mostly been settled, the dogma within the Children started to change. It started to warp into anyone that could channel was corrupted by the Dark One. This is when the first major shift away from the original teachings of Mantelar happened. Some think that this change in belief came from Aes Sedai once again becoming a political power to the world, but others think it is inevitable.

The Children were a volunteer army in times of conscription, but only those who were faithful rose through the ranks. From the thousands of men, only the zealots were the ones to reach the top, distancing themselves from the actions of the battlefield and delving into politics themselves. Once they had their stronghold on Amadicia, I would say the Whitecloaks began and the Children of the Light were left to the wayside until Galadedrid Damodred was named Lord Captain Commander near the end of the Third Age.