Ivan Pavlov was a Russian physiologist, psychologist, and physician. He was awarded the Nobel Prize in Physiology or Medicine for research pertaining to the digestive system. He is widely known for first describing the phenomenon of classical conditioning. The term “Pavlovian response” was coined after him. 
When asked if he was religious, Pavlov smiled and replied: "Listen, good fellow, in regard to [claims of] my religiosity, my belief in God, my church attendance, there is no truth in it; it is sheer fantasy. I was a seminarian, and like the majority of seminarians, I became an unbeliever, an atheist in my school years." Quoted in George Windholz, "Pavlov's Religious Orientation", Journal for the Scientific Study of Religion, vol. 25, no. 3 (Sept. 1986), pp. 320–27.