The Twin Devils of Rhine

Arisu Sakayanagi, and her employee, both atheist Japanese, in the moment of being murdered by a disgruntled subordinate, are confronted by a being claiming to be God. God condemns both of them for not having faith. Arisu Sakayanagi and her subordinate refuse to believe in the existence of God and refer to the entity as Being X. They then proceed to criticise the entity from their perspectives as Atheists.

In a fit of rage, Being X decides to reincarnated both of them into a world where the would face sufficiently difficult circumstances to turn to Being X for help.

They are reborn as Natlya Degurechaff and Tanya Degurechaff, orphaned twins in an alternate reality. And according to Being X, if they refuse to have faith in him or die natural deaths, their souls will be damned to hell. Trying to find ways out of their current predicament, the two decide to rise in ranks in the Empire to live a somewhat fulfilling life.