Your AHC: create, using any desired changes to OTL at any point, a United States in which the open practice of one or more religions is either permanently or (more likely) temporarily suppressed, for any desired reason.
Most likely this would be during some kind of national panic or state of emergency. Examples would be Islam getting declared haram after 9/11, or Asatru and/or Shintoism being banned during WWII. How hard would it be to make this happen? I suspect it wouldn't have taken much to prohibit the practice of Shintoism after 12/7/1941. I think a sizable minority (maybe 40%) of Americans would have been open to banning the Muslim faith after 9/11.
Bonus points if the Supreme Court signs off on the ban, temporary or otherwise.
MalcontentRex