Concerning religion, he was well known for vigorously defending atheism, and also arguing that the problem of evil made untenable the main monotheistic religions. His criticisms of the free will defence are particularly significant. He argued that the idea of human free will is no defence for those who wish to believe in an omnicompetent being in the face of evil and suffering, as such a being could have given us both free will and moral perfection, thus resulting in us choosing the good in every situation. In 1955 he published "Evil and Omnipotence", which summarized his view that belief in the existence of evil and an all-powerful, all-knowing and all-good god is "positively irrational".