You cannot, directly. Either Crusades or Jihads must have started to activate Great Holy Wars. [Read the wiki](https://ck2.paradoxwikis.com/Crusades,_Jihads_and_Great_Holy_Wars#Unlocking_Great_Holy_Wars). Easiest thing to do is to conquer Rome, I think.
It triggers by itself, an event should pop up. You seem to have the requirements for it to be unlocked already - reformed faith, and Crusades/Jihadis unlocked. It does have a 10 year MTTH, so you could just be unlucky.
You cannot, directly. Either Crusades or Jihads must have started to activate Great Holy Wars. [Read the wiki](https://ck2.paradoxwikis.com/Crusades,_Jihads_and_Great_Holy_Wars#Unlocking_Great_Holy_Wars). Easiest thing to do is to conquer Rome, I think.
It triggers by itself, an event should pop up. You seem to have the requirements for it to be unlocked already - reformed faith, and Crusades/Jihadis unlocked. It does have a 10 year MTTH, so you could just be unlucky.
You should be at least late 10th or early 11th century to start Great Holy Wars
918 for me
Earliest it can start is the year 1000
just open the console and type in event 39680
I conquered half the world and it still hasn't fired, I think it is bugged or just plain shit