Can we prove, philosophically, the existence of God?

This is something I've been thinking about today: can we, in a philosophical sense, prove the existence or non-existence of God?  It is my conclusion that we cannot, the existence or non-existence of God must come as an assumption, upon which further arguments may be built.

My reasoning is currently based on the central tenent of the modern, protestant christian religion, as I understand it.  The basic rule is that to be saved, per christian doctrine, one must choose to accept Jesus as one's own personal savior.  In essence, one must choose to believe in Jesus, and thus God and his various works, or choose not to believe in Jesus.  It is the requirement of this choice that I believe precludes the proof of the existence of God, as described by Christian doctrine.

If the existence of God could be proven then no sane human would ever be faced with a choice to believe or not.  No sane human would be able to deny God any more than they could deny that the sky is blue or that walls are difficult to walk through.