The best way to take down the Washtowel Slaveholdery is to expose the core essence of their doctrine as a scam. You will not be able to get those already in to see it, since they are programmed to worship the Filthful and Disgraceful Slavebugger. What you can do is to make it more difficult for them to get people interested.
If people in the territory realize that the whole concept of Original Sin is nothing more than a scam, they are not going to see the "need" for salvation from it. All the sacrifices that are required will be seen as a complete waste of time and resources. And no one is going to want to study the Washtowels if they can find better resources about Jesus (the true Jesus, not Paul's Jesus). And, it will be almost impossible for them to find new recruits to drag in if they all believe that Original Sin is a farce.
What happens next is that some will become discouraged. The Filthful and Disgraceful Slavebugger already addressed that by telling people not to get discouraged, and that is a sign that the end is imminent (as it was during the last 20+ years I was in). But, without new members, the money is going to get pretty tight. Publishers do not have bottomless bank accounts (and, once their credit hits max, there is no way they are going to have anything to donate to the Worldwide Pedophile Defense Fund). They will also have less money for field circus, and when the hounders tell them to keep going as if they had the money, some will stop on that.
True, this will also mean people will get out of other religions. But it will make it more miserable for the Washtowel Slaveholdery to gain new members. It will also make life miserable for the next humanoid that wants to start another religious cult--if people do not believe in Original Sin, the cults that play up Original Sin are going to have a miserable time trying to get people to fall for the scam. That, plus the "In a Sense" type "fulfillments(??)" being exposed as a farce, will make things almost impossible for new cults to start up.