Cain and Abel being from 2 different fathers is their (evil's) theory. They want Cain to be the son of Satan in order to convince themselves that they are from that bloodline. Later the Anak (Anunnaki) came down and bred with the human females in order to pass their genetics here on earth. Noah was without sin which meant he was free from the genetic disorder (giants, nephillim) bestowed by the fallen angels (Anak - Anunnaki). Noah's 3 sons brought with them each a wife on board the ship in order to populate after the flood. Ham and Japheth's wives carried the genetic disorder and Seth's did not. Jesus is from Seth's lineage.