Holy orders are military organizations that fight for the faith that they belong to. You can only hire them when you’re at war with a hostile or evil faith that another ruler belongs to. However, once hired, you can use them in any on-going war. To create a holy order, you will have to be at least a king and you will need to spend a lot of gold and piety.
This will then improve your relationship with the head of faith if there is one. Faiths can have multiple holy orders established, but you can only ever have one employed in your service at a time.
If you create a new religion or convert to a different one, you will lose access to any existing holy orders. Even the one you’ve established yourself.
Over time, grandmasters (leaders of holy orders) will seek to expand their organization and approach you in hopes of getting additional baronies (typically cities) leased to them. They will offer you gold in return, and if you agree to provide them with a city that they can use, their numbers will grow and expand over time.
Holy orders can grow and become very large armies and they also have men-at-arms as well as order knights which are powerful units.
Once you establish your own holy order, you will become its patron. This means that you can hire that particular holy order free of charge whenever at war with a hostile faith.
If the holy order is already hired by someone else, you can still spend piety and take control of it. You can also spend piety hiring other holy orders, but again, you can only have one at a time.
If there is an upcoming great holy war, all holy orders will be pledged to it automatically.