Christians can oftentimes be functioning atheists. We think we are immune to this particular moral failing. Thomas Ridgeley says otherwise:
There is a great deal of atheism in our hearts; by reason of which we are prone sometimes to call in question the being, perfections, and providence of God. The devil also frequently injects atheistical thoughts into our minds; which are a great affliction to us, and render it necessary that we should use all possible means for out being established in this great truth (Commentary on the Larger Catechism, volume 1, p. 9).