A wire service story writes of his audience with the pope: "Castro was educated by Jesuit priests as a young man, but later shed his religion for revolution, Marxism and atheism. Nonetheless, he bowed slightly when he met the pope and told him it was a 'great honor' to be in the Vatican."
To dispute this, I recently read something where Castro stated he would not divulge, whether he believed in god or not. Saying "He didn't want to offend believers, or non believers.". I have no doubt he is an atheist though.