[Morris70]

I think a short answer is that he was offended
To be called an atheist, he believed in a creator,
and did not believe in an afterlife.

[x]

What Einstein called "God" was the order, laws, and regularity of the universe. He was not an atheist, but he also was not a believer in a personal God. Like Spinoza, he identified "God" with the universe and its rational order. That at least is what I get from a quick search.

"I believe in Spinoza's God, who reveals himself in the harmony of all that exists, not in a God who concerns himself with the fate and the doings of mankind." -- Albert Einstein