A theme in science fiction just as much as it might be treated as a specialized subgenre (think Christian SF). Religion as a power force on mandkind has naturally been heavily explored by the science fiction genre.
Religious science fiction tends to focus on how religion, either a made up one reflecting some or a combination of ideas from present religions or a straight out recognized religion, extrapolated into the future affects humanity. But religious science fiction is certainly not just limited to this use of "religion" in science fiction. It can be used to critique a religion in the form of a dystopia or ask deep questions about a religion when you take humanity out of the equation (alien regions).