I think this question is very similar to esthers post about who God is.
Personally i would love to live a life without any doubts whatsoever about God existance but i'm human, i'm flawed and at times i do question. I also think questioning God's existance is natural and also an intelligent thing to do because it calls upon you to gain more knowledge and through this i believe each person will become closer to God. I think questioning will always be there, because afterall where would faith come in to play if we were 100% sure all of the time??

I was reading a very interesting article in the TIME magazine a few weeks ago about Mother Thereasa - it said in the article that papers and diary entries had been found from her questioning God and having a crisis of faith.

"""The letters, many of them preserved against her wishes (she had requested that they be destroyed but was overruled by her church), reveal that for the last nearly half-century of her life she felt no presence of God whatsoever """"

""""She compares the experience to hell and at one point says it has driven her to doubt the existence of heaven and even of God. She is acutely aware of the discrepancy between her inner state and her public demeanor. "The smile," she writes, is "a mask" or "a cloak that covers everything." Similarly, she wonders whether she is engaged in verbal deception. "I spoke as if my very heart was in love with God — tender, personal love," she remarks to an adviser. "If you were [there], you would have said, 'What hypocrisy.'"""""

Some the things written in her letters to various bishops really touched me and allowed me to relate to her pain. I'll write the refs to this article below as people may find it interesting too. The thing that touched me the most in this article, and i think makes her one of the most awe inspiring modern women is the fact that throughout all these doubts she remained in prayer, she remained doing the good that she did. I cannot even imagine having that strength and that faith.

http://www.time.com/time/world/article/0,8599,1655415,00.html