It got implemented about 2.5 years ago. You need a majority faith, but I forgot the exact percentage minimum. You also need at least a...large temple, I think?
The button was there 2.5 years ago, but I'd swear that the mechanic actually going into effect is not that old! That or time really did fly by... Hard to say.
You need 70% follower and a level 3 temple, I believe. The 70% marker might be variable, I think I've seen it available at lower percentages in some cases.
