Paul Ehrenfest (January 18, 1880 – September 25, 1933) was an Austrian and Dutch physicist, who made major contributions to the field of statistical mechanics and its relations with quantum mechanics, including the theory of phase transition and the Ehrenfest theorem.
1.) Ronald Clark (2011). Einstein: The Life and Times. Bloomsbury Publishing. ISBN 9781448202706. "That Einstein's attitude was the result more of muddle than agnostic scruple seems clear from a letter which he wrote less than two years later when Paul Ehrenfest ruled himself out from becoming Einstein's successor by roundly declaring himself an atheist."
2.) Thomas Levenson (2004). Einstein in Berlin. Random House of Canada. p. 172. ISBN 9780553378443. "The man he had hoped would succeed him in Prague, Paul Ehrenfest, refiased to compromise his true atheist's principles. Einstein scolded him. "Your refusal to acknowledge a religious affiliation" was just this side of "willful stupidity," he assured him, with the benefit of recent experience. Once he became a professor Ehrenfest could revert to unbelief."