Why should there be a whole complicated mechanic for this? If someone wants to play a Martin Luther they should found a new religion. If they can't get a single region lord on board (or if they're not a region lord themselves) then their doctrine clearly isn't powerful enough to gather followers.
If a spy infiltrates a religion there should be an option to ban them from receiving messages from the religion. It shouldn't be very difficult for church leaders to say "hey, that guy's a spy, stop sending him our mail". Infiltrating a religion can be a legitimate role played action, but to suggest hat spying should grant them a lifetime supply of free intelligence is ludicrous. Let them have the ability to send messages to the faith which should limit the Churches ability to use this as a way to summarily silence descent.
