I think it's obvious that existence can have a purpose apart from any belief in God in the sense that an individual who doesn't believe in God can certainly find reasons (often very good reasons) to keep on living. If we all simply die and go out like light bulbs, that doesn't mean that an individual's existence has no purpose or meaning to him and those around him or that his life isn't worth living. Some people would actually prefer that there not be an afterlife. Without some kind of theistic belief, however, there is no "larger purpose" to one's life - it has no meaning apart from its immediate context. Since someone mentioned William Lane Craig and I have just finished his Reasonable Faith, I will say that what I think the statement about "no purpose apart from God" is really referring to is the so-called Moral Law Argument for the existence of God, basically meaning that without God there can be no morality (see Craig's site at http://www.reasonablefaith.org/site/News2?page=NewsArticle&id=5344). In this sense, it does seem to me that a life premised on the existence of God is inherently more purposeful and satisfying than one without God, because with God life is viewed in a larger context and as having a deeper purpose. So I think it does boil down to the point that apart from God there is no common morality binding us together, no larger context, and no purpose apart from that which each individual finds for himself. (I'm not going to bog down in an argument with someone who insists "I can be just a moral without God as you can with Him!" No, without God there simply is no morality.)
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