Einstein is sometime quoted as saying, "God doesn't play dice." He is sometimes quoted as saying, "The Old One doesn't play dice." The second is more like what he's say.

He didn't believe in a personal God. He believed that the way everything fits together and works, and the beauty and complexity of how it works adds up to the Old One, whom he was playing to figure out, who he challanged and profoundly admired.

He thought that that Old Way, the whole way thing are when you understand them, doesn't play dice. He believed profoundly in determinism--this causes that causes that, inevitably. If you understood the chain of cause, you'd know the inevitability of all that happens.

Einstein's work was massively importantly in starting quantam physics, which he disliked and didn't belived.

Quantam physics says determinism is not what is happening at the smallest, most basic level. For Einstein, that went totally against his gut and why he got into physics.

So he spent the last decades of his life trying to prove that quantam physics, which he helped create, was wrong.

Niels Bohr, younger, said Einstein's intelligent opposition to quantam physics helped it develop faster. He asked good questions.

But after the part where he helped Bohr and others by challenging them, he got nowhere, because he was trying to oppose quantam physics. In a way, it might not matter, because in fields where high math is thick upon the ground, very few people do important work when they are older. So Einstein didn't do important work when he was older, in a focussed kind of way.

I think that thing where he got into physics because he loved determinism and then he helped destroy the determinism of physics, is the kind of thing the Old One does sometimes, and laughs.

--info from the book "Einstein and Religion"