Very few thinkers today would claim that the ontological argument proves the existence of God. Yet Immanuel Kant demonstrated how all arguments for God’s existence ultimately depend on this single, flawed idea. 

Kant is therefore an underappreciated figure in the history of atheism, and did nothing less than undermine the entire foundation of rational theology.  

The (Imaginary) God of Pure Reason: Kant on the Failure of Rational Theology