2.7 Vassal Holy Orders will fight any target.

[dkelly6]

I'm playing as the Emperor of Britannia in the 1066 start, it's now 1126 and I have managed to get my dynasty as the heads of both the Templars and the nights of Santiago (?). Now, I find they will literally fight any target, even other Catholics.

 

[Aardvark Bellay]

I'm playing as the Emperor of Britannia in the 1066 start, it's now 1126 and I have managed to get my dynasty as the heads of both the Templars and the nights of Santiago (?). Now, I find they will literally fight any target, even other Catholics.

They do under the appropriate circumstances*...for a while now. 2.6 i believe.
If you elaborate on the specific issue/situation and if you have mods, maybe there is more to it, but maybe not.

* not sure right now which these are, but someone else might tell...

 

[Serenity84]

It says this in the 2.6 patch notes:

"Fixed holy orders sometimes disappearing into nothingness in battles and sieges. Now holy orders will fight their own religion if owned by the person raising them, or a vassal of them"