Atheism and leftist ideology are not one, just as communism and leftist thinking are not the same even if they do share some common ground. 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 similarities with religion. Many leftists have sympathies for communist ideology because communists try to break the same status quo as the leftists. In the West, a galaxy of influential thinkers who later denounced communism started off by pledging allegiance to communism and the Soviet Union. The reason being that it was the most powerful ideology available that had the potential to break the back of many venal establishments like theocracy, feudalism and racism. It is another matter that it took a different turn when it demanded “proletariat dictatorship” and tried to suggest that individual liberty is secondary to the party/state/social interest.
My favourite however, 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 later went on to write a two-volume piece titled Open Society and its Enemies. He considered communism, along with many religions, as illiberal ideologies inimical to an open society.