Well, that's fine. But you're wrong. I do believe in conscience. Everybody has one.
But that doesn't mean a) everybody listens to it, b) nobody ever rationalizes it away, c) nobody ever damages it or loses touch with it, d) an evil society never dulls it by teaching people to do evil things and telling them that those things are moral and average, d) you don't have to explain to people why X or Y is good or evil, and e) that you can legitimately make laws or establish punishments without any appeal to objective moral standards.
So the fact that people do have consciences doesn't nearly address the problem of establishing terms for morality. The mere existence of an "innate moral sense" manifestly is not enough to keep people moral. That's empirically obvious, too, because if it were not so, nobody would do evil, and nobody would call anything "good' either, because neither word would have meaning.
what else do we really have, though?
all justifiable convention, law, codification, roots in the moral sense
you can point to revelation (scripture, for example), god's word; I'll counter that revelation, if real, was still conducted through man, an imperfect, willful creature, prone to interpreting
you know my spiel: god created, is creating: that's the extent of his involvement; he built man to get along without his direct guidance...to that end: each man has a compass that always reveals true north...as a free will , man gets to attend to the compass or ignore it
the question, then: is morality, as you and I see it (as moral fact) adhered to more clearly with revelation, or would the world be pretty much the same without revelation (with only man's moral sense to rely on)
I think the world would be different without theism, but not appreciably better or worse
to be clear: revelation, from my place, is man's attempt to codify the moral sense, not god codifyin' through man