I doubt he is immortal, so we can't call him a god in that sense. I think Sephiroth tried to become a god by trying to absorb all of the planet's Lifestream in FF7. I think he successfully managed to absorb at least 1/2 of it (the other 1/2 was controlled by Aeris/Aerith I think), and would have succeeded to absorb all of if Meteor had hit. But since he failed, I don't think he is a god.