The question of whether a church is legally a public or private
space for the purposes of determining whether a release is necessary
has probably been addressed by some court somewhere. My guess is
that the stronger argument is that worshippers have a reasonable
expectation of privacy in the interior of a church which cant be
seen from outside and that their permission to be filmed should be
secured. But pretty much anything done in public or which can be
seen from the outside ie through a window is fair game.