If you lose Religiousness/Cutlure/Defense points, yes, you must theoretically lose territory to compensate.
However, I do not believe this will be a common occurrence. Typically, it is difficult to lose Defense and Culture points that you have already gained. It is possible to lose Religiousness, but it is also entirely possible that by the time you lose points in Religiousness, your Defense and Culture points will rise high enough to compensate for the loss.
Also, I don't intend to implement a strict "if you lose points, you lose the territory" sort of policy. Instead, I encourage nations that face this situation to RP a civil war, civil unrest, or some other destabilizing RP scenario.