I think the very fact that there are so many points of view on this very topic (Celie writing to God, I mean) even within our own group is why Walker wrote it that way. It's somewhat ambiguous, and lends itself very well to discussion. What is Celie's need for God? Who is she really writing to? What IS her religion, if any? These are all questions we are asking, and as we ask and think, we face our own thoughts and beliefs, and I think that's the point.
It appears to me, that praying/talking to God helps her talk to someone. It is something horrific and she has to talk to someone to keep from going crazy. God appears to just be a release.
I believe that every woman has huge inner strength! sometimes we just can't realize it!