The basis for much of
existentialism and
postmodern thought. Once you are exposed to people with different beliefs from your own, it becomes hard to avoid the realization that people find meaning for their lives in a very wide variety of ways.
The logical conclusion is that meaning is subjective, defined by each individual for himself or herself, rather than objective, set down by a power greater than ourselves.
Many apologists try to claim that it is possible to use empirical methods to arrive at the meaning of life, but they're wrong...and these attempts lead to bad theology. Any of these arguments eventually run into the problem that human beings will believe anything, and press a dizzying variety of ideas and things into service as the meaning of life. Objectively, it isn't possible to show that any one of them is clearly the right choice.