Reborns are simply just Imperials, probably were once Stormtroopers or Imperial Officers, who were taken to the Valley of the Jedi. There, they "stole" the Force power from the temple and were given a lightsaber. They probably went through some kind of Force training after they soaked up some Force.
Cultists are very similar, but instead of getting their Force from the Valley of the Jedi, they are empowered from the energy which was stored in Ragnos' scepter. They are constantly feeding off the scepter's energy, so when the scepter was destroyed (in the light side ending), their energy source was cut off, therefore they lost their power.