You are a mortal. That means you have free will. Gods respect that, that’s why they will never force anyone to be a paladin. (Gods can only truly interact with mortals if it’s fated. when it happens it’s divine intervention)

Being a Paladin is about believing in doing the “right” thing (wether that be fighting Chaos for Justicars or torturing fair virgin maidens for Villains) if you lose faith, you lose your powers. That’s all there is to falling. I, as the GM, will never let your character lose faith, that’s for you as a player to decide when/if it happens (e.g. if a demon offers unlimited power if you renounce Mitra). I’m merely the temptator.

When you are a Paladin, but haven’t done your “signature move” in a while (fighting chaos / promoting law in your case), the church and other agents (Justicars, Priests, Angels, AnzĂ») will come and demand you do all of that. They might kick you out of their church, but you will not necessarily lose your powers (if your faith can survive being told off by an angel)