The Abraham Lincoln Brigade was made up of volunteers from all walks of American life, and from all classes. Many of the people who volunteered for the Abraham Lincoln Brigade were official members of the Communist Party USA or affiliated with other socialist or anarchist
and are there other...
Karl Marx famously called religion the "opium of the masses" and thus made his made his theories more attractive to individuals of the atheist persuasion
You don't find too many God-fearing commies, nor many free-market atheists. There are exceptions to every rule, of course.