From Walter Isaacson's recent biography of Einstein:
For this reason, Einstein had a terrible time accepting the idea of quantum mechanics. It differed with his idea of ultimately knowable and imutable laws of nature -- hence his idea that God did not play dice with the universe by allowing events to be random or undetermined. Quantum physics shows Einstein's overall view to be incorrect.
Einstein embraced his [Spinoza's] concept of an amorphous God reflected in the awe-inspiring beauty...of nature's laws. But like Spinoza, Einstein did not believe in a personal God who rewarded and punished and intervened in our daily lives (p. 84).
For this reason, Einstein had a terrible time accepting the idea of quantum mechanics. It differed with his idea of ultimately knowable and imutable laws of nature -- hence his idea that God did not play dice with the universe by allowing events to be random or undetermined. Quantum physics shows Einstein's overall view to be incorrect.