No more than we take *your* existence as a provisional hypothesis. I have as
much reason (probably more, 'cause I've never communicated directly with you)
to believe in God's existence as I do to believe in William Provine's or
Massimo Pgliucci's -- and the reasons are, in some respects, quite similar. I
don't find many of the classical arguments for God's existence very convincing
-- though some are persuasive -- but I am nonetheless convinced because I have
had personal contact with Him. Absent contact of some sort, I wouldn't have
much reason to believe in Will or Massimo, either.
I have considerably more reason to *have faith* in God than I do to have faith
in Will or Massimo. This is not an insult or a slight. I have the same
reasons to have faith in God as I do to have faith in my wife: long
acquaintance and knowledge that she loves me, as demonstrated over and over
again in concrete ways.
If I have an argument with my wife, I don't abandon her, because I believe in
her. In the same way, I don't always understand how God is working or what He
is doing, but because I believe in Him I try to stay with Him.
Christians have a personal relationship with God. Now, a personal relationship
*is* greatly aided by the provisional hypothesis that the one with whom you are
relating may exist... but not much. If I took my wife's love for me as a
provisional hypothesis, our relationship would probably soon end. If I took
her *existence* as a provisional hypothesis, she'd probably whop me upside the
head for a convincer... God does that too, once in a while.