While I haven't done an ounce of research on the subject, I doubt that there is anyone who doesn't believe in conscience. Or at least some form of inner compass. Even so-called savage peoples have a degree of ethical behavior. There are some things they just won't do for fear of losing their lives. I would dare say that there are espousing Atheists who believe whole-heartedly in such a notion. They will tell you that it is their conscience which guides them in their choices, not God. Which I find to be ironic since it holds up to none of the empirical evidence they seem to need in order to believe in God. You can't touch it, see it, talk to it, dismantle it or point to its molecular structure or a mountain of data to prove it. Has anyone ever held a conscience in their hands? No. You just 'know' it exists inside you. I guess you could say it takes faith. And if there is a God then the world would not be so evil. To them I would ask this question then: "If conscience exists then why is the world so evil?"
The answer is quite simple, actually. You can't impose your conscience upon (or inside) another. People will either ignore their conscience or override it through rationalization. I think that makes perfect sense. People with God in their heart do the same thing all the time. You cannot impose God on another soul, you can, however, love them and point them in God's direction. I don't do that often enough. And that, my friends, is why the world is so evil.