Well, this is a conflict talked about by Nietzsche, that when one falls out of the christian faith, he falls into nihilism, causing cognitive dissonance. Certainly those who were devout and fall out of the faith, go through some extreme mental anguish to try and make sense of the world again without god. It takes time, but with it, comes the realization that god never existed, and none of it ever was true. it was just a fiction created by the mind. So, we are left to give our lives whatever meaning we ascribe to it, which is actually no different that a religious person's, its just that devout believers believe that god has put a stamp of approval on whatever purpose they choose or follow, but really, it is the same. You give your life meaning by what you value. If you value friendship and family, then that gives you're life meaning: relationships with others. If you value your vocation, then that gives you a purpose, and you might see it as a way to connect to, reach, and hep others. Many atheists and theists, have not found a purpose, and so search for their passion in life. I don't think this has anything to do with god, in either case. It is simply a theists beliefs that god is watching that he thinks gives him purpose, but this is only a belief, and although it may make him/her feel good or vindicated in whatever he/she chooses, it doesn't make it reality. So, in the end, we are all living for ourselves, hopefully to help others in some way. This dichotomy of altruism/selfishness is a complex one, but useful. We may all be intrinsically selfish, in that we want to feel good, but the paradox is that part of feeling good is forgetting what we want, and thinking about others. So, there is a built in altruism, that doesn't require god. It can be evolutionarily explained, and is something we all possess the capacity for, and gives our live meaning, because it connects us to others.