i'm also not christian, but souls in the buffyverse aren't a religious thing, they're a morality thing. because of his love for buffy, spike developed a sense of morality through trial and error, and he felt terrible if she or people she loved got hurt. he got his soul back when his version of morality seemed to not be enough. i agree that the soul didn't change his capacity for love or compassion for those he loved, but it did give him a much bigger conscience - instead of just feeling bad for hurting buffy, he also had to deal with guilt for all the people he'd killed since the 19th century. so the soul wasn't very good for his mental health, but it definitely meant something.