Einstein credeva in un universo deterministico, retto da leggi immutabili, e come tale incompatibile tanto con l'idea del libero arbitrio:
"Human beings in their thinking, feeling and acting are not free but are as causally bound as the stars in their motions"che con il concetto di un Dio personale, che si preoccupi dei destini o delle azioni dell'umanità e intervenga sul corso degli eventi:
"The main source of the present-day conflicts between the spheres of religion and of science lies in this concept of a personal God"D'altra parte, Einstein non si definiva ateo:
"I'm not an atheist. I don't think I can call myself a pantheist. The problem involved is too vast for our limited minds. We are in the position of a little child entering a huge library filled with books in many languages. The child knows someone must have written those books. It does not know how. It does not understand the languages in which they are written. The child dimly suspects a mysterious order in the arrangement of the books but doesn't know what it is. That, it seems to me, is the attitude of even the most intelligent human being toward God. We see the universe marvelously arranged and obeying certain laws but only dimly understand these laws."La posizione di Einstein era complessa, estremamente personale, e difficile da ingabbiare in una definizione, o irreggimentare in una chiesa. La risposta più sintetica alla domanda diretta se credesse in Dio fu la famosa frase:
"I believe in Spinoza's God, who reveals himself in the lawful harmony of all that exists, but not in a God who concerns himself with the fate and the doings of mankind."L'irriducibilità di Einstein gli provovò critiche e attacchi tanto dai sostenitori delle religioni tradizionali che dagli atei militanti. Evidentemente, ad Einstein non piaceva essere tirato per la giacchetta:
"There are people who say there is no God. But what makes me really angry is that they quote me for support of such views."e ancora:
"The fanatical atheists, are like slaves who are still feeling the weight of their chains which they have thrown off after hard struggle. They are creatures who--in their grudge against traditional religion as the 'opium of the masses'-- cannot hear the music of the spheres."