Novela com ciência disfarçada de religião; rebeldes usam o ocultismo para lutar pela liberdade

Recolha, Trevas! (1943) de Fritz Leiber, talvez? Serializado pela primeira vez em Ficção científica surpreendente, Maio a julho 1943, e disponível on-line no Internet Archive (parte 1, parte 2, parte 3).

Esta revisão em Goodreads menciona o Grande Deus e as bruxas:

Three hundred years after a nuclear holocaust, humanity is ruled by a secretive religious brotherhood, the Hierarchy of the Great God, which uses science as the supernatural means of ensuring its control over an impoverished, predominantly illiterate population. Opposing this brotherhood is a secret society of witches, the followers of Sathanas, seeking to use science to overthrow the brotherhood's tyrannical theocracy. A young Hierarchy priest, Brother Armon Jarles, skeptical of his beliefs and the Hierarchy's orthodoxy, soon finds himself caught in the bitter struggle between both groups. A struggle that is meant to gather darkness against the Hierarchy's grip on power.

Outra revisão menciona o "Grande Deus" produzindo milagres:

Written in the 1940's this novel postulates a future where the Hierarchy control the populace by using religion as a control. Using science to produce false "miracles" the Hierarchy brainwashes the populace into obedience--the more clever of the 'commoners' are encouraged to join the priesthood.

So, when an group of revolutionaries begins to overthrow the tyrants, the rebels disguise themselves as "witches" and use their science to create miraculous effects ascribed to "Sathanas".

Encontrado com a consulta do Google livro ciência religião "grande deus" site "rebeldes": goodreads.com que retornou a página Goodreads, destacando a segunda revisão.