I realized recently that out of the five Star Trek series, the majority have religious first officers.
Original Series – Spock doesn’t talk about religion and probably wasn’t meant to have a religion at the time of writing. But later on Vulcans are definitely portrayed as religious, albeit in a very un-Judeo-Christian way (e.g., Tuvok). Seeing as Spock is pretty keen on being as Vulcan as possible, we can safely assume that he was religious.
Next Generation – To my knowledge Will Riker had no religion, but I could be wrong.
Deep Space Nine – Entire episodes revolve around Kira Nerys’ religious beliefs.
Voyager – Chakotay’s generic but genuine Native American spirituality is sympathetically portrayed, though the writer’s seem to have completely forgotten about it halfway through the show.
Enterprise – T’Pol is Vulcan, and is portrayed as engaging in spiritual practices to a greater degree than Spock.
So that’s four out of five, or three out of five if we don’t count Spock; in any case a majority. If we do count Spock, Riker is the lone secular exception. If we cheat and count Spock twice because of the Animated Series, that’s five out of six.
There’s also an interesting mix of beliefs- polytheism, ancestor veneration, nontheistic philosophical mysticism. The lack of monotheism is notable, though Bajoran religion is obviously imitative of the Abrahamic faiths.
Of course, other than Ben Sisko (sort of), we have zero religious captains. Kirk expresses belief in God, but he might have been rhetorical or sarcastic.