Not all that sure? C S Lewis

Just as the Christian has his moments when the clamor of this visible and audible world is so persistent and the whisper of the spiritual world so faint that faith and reason can hardly stick to their guns, so well as I remember, the atheist too has his moments of shuddering misgiving, of an all but irresistible suspicion that old tales may after all be true, that something or someone from outside may at any moment break into his neat, explicable, mechanical universe.

Believe in God and you will have to face hours when it seems obvious that this material world is the only reality; disbelieve in Him and you must face hours when this material world seems to shout at you that it is not all.

No conviction, religious or irreligious, will, of itself, end once and for all this conflict in the soul.

Only the practice of Faith resulting in the habit of Faith will gradually do that.

          --From 'Religion: Reality or Substitute?'
                  in Christian Reflections, by C S Lewis