Orwell was a critic of authoritarianism, detesting both Adolf Hitler and Josef Stalin alike, and finding similarities between Nazism and Stalinism: the rich control everything and the poor are weak, and war is something that happens regularly. Sound familiar? It's not clear what Orwell's religious beliefs were but he was suspicious of organised religion.
He also fought in Catalonia during the Spanish Civil War as part of the Republican faction, where he was a member of the Workers' Party of Marxist Unification.
As long as supernatural beliefs persist, men can be exploited by cunning priests and oligarchs, and the technical progress which is the prerequisite of a just society cannot be achieved.