what do christians (and people in general) mean by "feeling empty inside"?

[Whateverist]

I think the idea of a "God shaped hole" or that feeling of emptiness inside likely points to a phenomenologically real thing, that there are specific subjective states which feel like that, though looking at responses such as GC's, I suspect it's used most often simply as an apologetic "stub" by people who may not ever have even had the experience. I suspect if you asked 100 Christians to describe the experience, you'd get all sorts of descriptions of anything from the mundane feelings of sadness to the sublime heights of existential angst, such as those pierced by Keirkegaard and Christian mystics. Various religious thinking triggers brain functions associated with interpreting the existence of others, other minds, others' intentions, and the self. (e.g. Jesse Bering's work on the belief instinct, and the observation that fMRI scans show that when a person is thinking about what God would want, they are activating the centers that activate when thinking about what they would want.) So I suspect it's a real phenomenon, seemingly related in a way to Sartre's look of 'the Other', but I doubt the Christian interpretation of its metaphysics and psychological meaning have any validity. But then, that's typical of religion, and especially modern Christianity, to take a powerful emotional experience and graft it onto an existing metaphysic, thus satisfying the "sounds reasonable" center of people's brains. (As a side note, Buddhism is guilty of much the same thing in its approach. It takes common emotional experiences and welds a conjectural psychological and metaphysical explanation onto those experiences, to encourage adoption of the religious practices [from meditation to specific ethics and politics].)

I agree that institutional religion does exist to swap out interpretation for direct experience. We are spared the angst of interpreting our own experience in exchange for helping to defend the shared alternative.

There is plenty to be in awe of without any particular religion's pantheon and story line. I think the "god shaped hole inside" may just be our sense that our conscious self is always a small thing beside the greater otherness within. Some how within the dynamics of my psyche/mind/soul I experience intuition, insight, conviction, inspiration and even such mundane experiences as reaching for and finding the right word when looking to express an idea. We also experience times of dullness when we are uninspired, lack understanding and can't quite find the words to express what we mean.

I think living with this mystery beats calling it by any particular religious system's names and accepting their institutional interpretations. If you're willing to do without certainty -which is illusory any way- and if you can accept yourself warts and all, then you can live with the mystery and make your own way. Seems like the only game in town worth playing to me.