But until he actually attempts to go to the church with matches and gasoline, the FBI can’t do anything – that’s speech, and (IANAL) unless spoken publicly with a threat to intimidate, protected.
Now, if the FBI talked him into going to the church with a fake bomb, then they could apprehend him and get him off the streets.
Is that what really happened here? I have no idea. (See posts with the word “ambivalent” in them.)