Sarah, I would suggest that Christians act as though the existence of God were a matter of assured knowledge simply because this is the sort of faith or belief that requires embodiment if it is to have any integrity at all. That embodiment, moreover, is unlikely to remain purely personal. We are a naturally social species, so faith becomes embodied not merely in individual behaviour but in corporate behaviour: religious, social and political practices.

By that point, however, we have got a long way from the original intuition by faith that God is rather than isn’t. Arguably, the New Testament is the story of how Israel recovered that original intuition, as a community, under the extreme conditions of the ‘wrath’ of God, manifested in the form of Roman occupation and imminent war. Paul argues in Romans, I think, that the survival of the descendants of Abraham hung on the seminal pistis - belief, faith, or better faithfulness - of Jesus, who demonstrated quite concretely that the people could genuinely trust the notion or story of the Creator God that they had allowed to shape their corporate existence ever since Abraham decided to listen to that voice in his head that said he could make a difference. The story of Gethsemane shows that even for Jesus it was by no means an easy thing to translate faith into commensurate action.

So I think you’re right. It is simply belief that lies at the bottom of this whole thing, but there appears to be something about the nature of human existence that makes belief in God for some, perhaps for most, inescapable - for all our rationalism, humanity remains captivated by the idea of something beyond our knowledge and power and wisdom, beyond our material existence, beyond the corruptibility and decay of the world. There is then the question, of course, of why this story of faith rather than another one. But for now, I would almost characterize the ‘Christian’ calling as being precisely to give communal embodiment to the believed story of a God who makes new his creation through love.

There have been a couple of recent discussions initiated by Jacob that may be relevant to this topic;

Faith and evidence
The Irony of Theological Realism, or, The Realist’s Dilemma