I wouldn’t say the RSF is atheistic, but it’s a secular group, in a country which was islamist until 2019. Officially, the regular forces are secular too, as the religions and the State were separated after the protests of 2018-2019, so it’s not the main difference.

I’d say it’s mainly a fight between two dictators who tried to form an alliance but were not capable of sharing the power. There’s a little more ideological diversity within the regular forces though, as the RSF is deeply and strongly Arab-supremacist.