Growing up my mother always labeled herself agnostic but she was really what I would call a 'recovering Catholic.' One of the most telling signs of her Catholic upbringing was the guilt which permeated everything she did. I've heard, of course, about the legendary 'Catholic Guilt' which people joke about and talk about almost flippantly, but I think it's important to really take a look at guilt and what a powerful weapon it can be in the hands of religious leaders.
It has been said by religious leaders of all denominations that guilt, being the feeling of responsibility or remorse for an offense - real or imagined, is the natural feeling we all have because of sin. In fact, some religious people would go so far as to say the human propensity for guilt is somehow proof of god's existence. Why else would we be plagued by this terrible emotion?
From an evolutionary standpoint, guilt can be considered a valuable trait as it aids people in engaging in and maintaining social relationships and thus benefits the species in regard to reproductive success. It is important for people to have an innate negative response to mistreatment of others within the group - ie guilt, remorse, shame, etc. - in order to facilitate living in groups. It is also important to note that while modern man's social and intellectual evolution have lessened the importance of existing in a group, this was once necessary to the advancement of our species.
If you run across someone who does not feel guilt or remorse you tend to refer to that person as a sociopath and treat them like an unstable and often dangerous entity. This is because when someone lacks guilt, it creates a completely different list of acceptable behaviors for that single person which is not common to the group and these behaviors often include mistreatment of others which would normally be kept in check by the inherent existence of guilt, among other emotions.
Guilt is powerful because it's one of the emotions humans (among other animals) feel in order to maintain social balance. The fact that religions have tried to use the existence of guilt to justify and in some cases prove their beliefs seems little more than human nature, but to confuse the human need to understand emotions through simplification - often in the form of religious explanation - with some kind of truth is very dangerous. We are gaining a better understanding of ourselves as a species every day, and I look forward to a time when we understand enough of why we think and feel the way we do to not need a blanket response like 'religion' or 'god' as an easy answer for whatever we experience which we can't easily explain or understand.