Atheism and leftist ideology are not one, just as communism and leftist thinking are not. They have common grounds, but they are not same. Atheism is just a stepping stone for a wider skeptical view of the world. It’s just an initial baby step. Leftists have a liberal disposition, therefore they tend to pass the same route atheists take. Communism has similarity with religion. Many leftists have sympathies for communist ideology because communist try to break the status quo the leftists wish to demolish. In the West, a galaxy of influential thinkers who later denounced communism started off with allegiance to communism and the Soviet Union. The reason for this was it was the most powerful ideology available that could break the back of many venal establishments like theocracy, feudalism and racism. It is a different matter it took a different turn when it demanded ‘proletariat dictatorship ‘ and tried to consider individual liberty secondary to the party/state/social interest. Bertrand Russel is one example. My favourite is someone not that popular- Karl Popper, philosopher of science, known for the influential ‘falsification doctrine’ of how science works. He started off as a communist in Austria, but would later write a two-volume piece titled ‘open society and its enemies’. He considers communism together with many religions as an illiberal ideology that is inimical to an open society.