Not believing in God

God has to work in the soul in secret and in darkness, because if we fully knew what was happening, and what mystery or transformation God or grace will eventually ask of us, we would either try to take charge or stop the whole process. No one oversees his or her own demise willingly, even when it is the false self that is dying. God has to undo our illusions secretly, as it were, when we are not watching and not in perfect control, say the mystics. We move forward in ways that we do not even understand and through the quiet workings of time and grace, as ‘Deep calls unto deep' (Psalm 42:8)' Anonymous 
Both you and the new man must walk day by day in the path of spiritual progress. If you persist, remarkable things will happen. When we look back, we realize that the things which came to us when we put ourselves in God’s hands were better than anything we could have planned. Follow the dictates of a Higher Power and you will presently live in a new and wonderful world, no matter what your present circumstances! (Big Book)
A very common phenomenon: Not believing in God, because none of the conceptions apparently available 'fit'.

To be aware of: Any knowledge of God cannot be conveyed. It is literally unspeakable. 
7 Revelation is literally unspeakable because it is an experience of unspeakable love. (ACIM)
Any approximation will be bloodless. God must be experienced. Once God is experienced, it's impossible to even begin to convey what this is like and obvious folly to even try.

The danger: Dismissing the notion or affirming one's 'atheism' in the light of the above. Doing so blocks the channel to experiencing God and creates a Catch-22 situation.

Instead: Open the mind completely and act to contact God.

What's the most definite method? Petitionary prayer, gratitude prayer, and affirmatory prayer (try Emmet Fox, Ernest Holmes, and A Course In Miracles in particular for the latter); take the spiritual ideas and dwell on them. This is meditation. Meditation is about changing conscious beliefs and thought patterns, in addition to practical entreaties to God, in order to change subconscious / unconscious beliefs and thought patterns, in order, ultimately, to produce altered manifestation in the material realm.

If you do this at definite times, at pitstops during the day (e.g. walking to the kitchen), and whenever temptation arises:
The consciousness of your belief is sure to come to you. (Big Book)

 The central fact of our lives today is the absolute certainty that our Creator has entered into our hearts and lives in a way which is indeed miraculous. He has commenced to accomplish those things for us which we could never do by ourselves. (Big Book)