We are all descendants of Seth, not Cain.

 The details are important. 

I had wrongly believed we were all descendants of Cain, just as I once wrongly believed Noah brought two of each kind of animal on the ark, not the 7 of some he did, and just as I once wrongly believed Moses was asking Pharoah to free his people from slavery, when he was asking the King of Egypt to let them have a festival to worship God in the desert. The Bible also records all the Egyptians, except the Israelites, selling themselves into slavery to Pharoah, for food during the seven years of famine, according with the historic fact that the Egyptians were forced to work by Pharoah between planting and harvesting their crops. 

I think someone wrongly told me that, via Noah, we were all descended of Cain. But the bible actually details all of Cain's descendants being wiped out in the flood, along with all of Seth's descendants who intermingled with Cain's descendants and were corrupted by their seemingly sinful ways. 

Noah is a descendant of Seth, the child Adam and Eve had to replace Able, whom Cain murdered. We are all descendants of Seth, whom the bible deemed to have been holy and whose descendants were deemed Sons of God, versus Cain's who were deemed Daughters of Man. While fallen, and born with original sin, that of Adam and Eve's pride, we are truly descendants of a noble line, Seth's, not that of a murderer, Cain, whose entire line was wiped out in the flood.