Conscience is the mind's structure, process, and outcomes that relate to moral reasoning. It is a person's capability to engage in moral reasoning. It is an innate structure of the human person. As such, it is part of what philosophers call Natural Law. And as per Steven Pinker, Harvard professor of cognitive psychology, the brain is hardwired for morality.
Counter examples of people without conscience do not disprove the general claim. There are exceptions (psychopaths, sociopaths), but these are examples of people with deficiencies and not the normal case, just as people with physical diseases do not disprove propositions about what a healthy human is like.
One corollary of the above is that one does not need to be religious or a philosopher to be moral.