What makes life meaningful?

This is a discussion in the Stanford Encyclopedia of Philosophy on the meaning of life.

Some, like Craig, argue that life is absurd and meaningless without God. I guess I am more interested in a different type of issue. If atheists find their lives to be meaningful, what does it mean to tell them that, no, your life REALLY isn't meaningful. A more interesting question might be whether the possibility of finding a meaningful life is possible for everyone, or whether some people, in virtue of their circumstances, can't find one. It seems to me that the circumstances of a particular life might result in someone thinking their life has no meaning, and Christian theist might offer Christianity as a solution to this difficulty to everyone, where as the viability of atheistic solutions is contingent on circumstances.