The fool hath said in his heart, There is no God. They are corrupt, they have done abominable works, there is none that doeth good. Psalm 14:1-7
At some point a Christian must decide whether or not they will actually believe what scripture says. When you come up against a verse like this, the tendency for us is to see exactly how we can make it fit into our current way of thinking. The flaw in that approach is that you never get to see where the line of thinking takes you when you take scripture at it’s word.
A fool is not someone who lacks intelligence. They are often the intellectual, even more often they are the Christian engaged in sin. If I want to steal the jelly donut and I know that God has said it is wrong to do so, God’s rules become an impediment on what I desire. I then have a choice about what I will say in my heart. I’m reasoning from desire, not what I know to be true.
I want the donut, to hell with God.
Christian or Atheist, apart from God no one can do good and it all starts in the heart.