Revolutionary France tried creating one not once, but twice: see the "Cult of Reason" (https://en.wikipedia.org/wiki/Cult_of_Reason) and Robespierre's "Cult of the Supreme Being" (https://en.wikipedia.org/wiki/Cult_of_the_Supreme_Being).

And of course the cults of personality that sprang up around Stalin in the Soviet Union (https://en.wikipedia.org/wiki/Stalin%27s_cult_of_personality), around Mao in Communist China, and in various Communist client states around their own local tinpot Stalin would probably qualify as well.

This is the problem with killing God: it's really hard to find a healthy candidate to replace him with.