I have given up on religion for a few reasons.
One: I've examined the facts and the evidence around me, and from that, I can draw a conclusion for myself, and myself alone, that almost everything religion teaches is a lie.
Two: I grow up in a religious household. I innately dislike authority and being told what to eat, what to believe, who to marry, and who to keep as friends really fuck with that innate thing inside me. I naturally rebelled and shit has not been the same.
Three: I see what religion does to people and I can't help but be sick. I mean, if you trace it back, you can argue that almost every war ever fought comes down to religion and that is horrible.
The thing is, religion without a god is cool. Basic Christian, Jewish, Islam, Hindu, and Buddhist principals are the shit. They all tell you to love thy neighbor, to not kill, to not cheat, to not be a dickwad in general, but once they throw god in there, shit goes insane and everyone kills eachother. Not for me man.