I know one spec fic that is called Sci-fi, but the science is so iffy that I tend to think of it as retconned fantasy. The author tried to leave out religion. She replaced it with an aura of mystery around one of the social groups on the planet, and all the oaths they have revolve around that class and the threat they face. In the end, she set up an unorganized cult, or at least that was the feeling of it for me by the end of the first few books.

That does not, however, mean it cannot be done. Depending on the level of rational thought each society of your world has, it could be very feasible. Or maybe it's there, but doesn't subsume the people's thoughts and interfere.

The most important thing is for you to feel comfortable with your world. If it feels right without religion, it's right.