If I may, you're going to lose this issue. Just the battle, mind you, not sure about the war.
I love the idea. It needs to be done, but it wont be done today. As of my writing this, the petition's already been up for around 10 days and only has 53 out of the 25,000 necessary signatures. Unless there is some kind of boom advertising this message, it won't meet the cutoff date, unfortunately.
Cults hid behind theocratic beliefs to keep the money they con people out of. It's a shame. In the WTS' case, they'll argue it as unconstitutional since they can angle the Scriptures any way they need to. See, recognized members need to believe the current doctrine, otherwise, they can't claim tax exempt. After all, exempt status comes from the size of the congregation and if the WTS was trying to claim Catholics as WTS followers, it'd be fraud.
So they shun (disfellowship). When you are disfellowshipped, you are no longer one of the congregation, and they have the paperwork to prove it. This is a major reasoning in why those who disassociate are put in the same regard as the disfellowshipped ones. The major reason disassociated people need to write a formal letter to be recognized as such.
If there was no paperwork, someone calling themself a Jehovah's Witness who believed different doctrine could shift their whole exempt status. This is where faders are most prominent. Unfortunately, if any faders came forward, the WTS would immediately disfellowship them under grounds of Apostacy or some crap like that, long before the motion to investigate the Society's claim ever begins.
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You need to advertise wherever you can. The next petition to remove exempt status from cults needs to be a consuming movement. You'd need to identify yourself as a Christian to every person you introduce this petition to so they can understand you're aiming specifically at radical fundamentalist cults. Not mainstream religion.
Facebook, twitterbombing with well-organized hashtags, flyers, etc. Go to Kinkos or some copy store and make a petition yourself. Go door to door with it. This cannot be won easily with everyone staying home.
You're essentially asking people to move a mountain with a puppy and a sled. You need to equip people with the right tools and motivation and THEN you'll get your 25,000 signatures. But not this time. You need to prepare that this time is a losing battle. Prepare for the loss and move on to prepping the next battle.