This article discusses the main points in the Bertrand Russell and Frederick Copleston’s debate about the existence of God. Copleston defended the cosmological argument based on a sufficient reason and argued against radical contingency in explaining the origin of the world. During the debate, the understanding of necessity was discussed, whether the word ‘God’ is a proper name or a description, whether the universe as a whole can have a cause, and the arguments about the origin of the world formulated in modern physics. The whole debate is an excellent example of the difference between a theist and an atheist with regard to Leibnizian type of the cosmological argument.i