My guess is that going to Latin Mass (which is not a mainstream Catholic thing post-Vatican 2) is a reliable proxy for belonging to one of those ultraconservative schisms, quite possibly one which denies the legitimacy of any recent popes, which in itself may be a proxy for other extremist views. Also, this sticker predates the change of regime in Washington, and comes from a time when the FBI was thought to keep an eye on right-wing extremists.