Just remember, not all people who claim to be Jewidh, are descendants of Jacob. Some are descendants of Esau. The Romans forced some Edomites to become Jews. Edom is probably one of Israels biggest enemy. Take a look at ancient maps. Where was Idumaea? Who lives there now?
God made His promise to Abraham, not Cain though.
Besides, Cain was BEFORE the flood, so unless one of the six people on the ark were of his line, no one exists from it today, at least not genetically, spiritually is a different matter.
Yes, it's possible, but not likely. Cain was cursed and was marked to prevent others from killing him. He and his wife moved away. That suggests to me that most people probably avoided them.
You first.
Ever wonder why the Romans changed the name of the middle east region to Palestine? Where did the Philistines live? Palestine is the Greek version of the word Philistine. The Romans got so fed up with Israel rebelling, (and succeeding) that they destroyed Jerusalem and renamed the region Palestine as an insult because they knew history and that the Philistines had been one of Israels main enemies in the past. The Edomites, descendants of Esau, cheered every time Israel got their asses kicked. They still do it today. Guess who many of the so called Palestinians are today? Who are they descended from? It's a mixture, but who DIDN'T get whacked? Guess who moved into the territory vacated by the Israelis after the destruction of the temple etc? Israels ancient enemies?
The Romans, during the time of Christ, had placed an Edomite ruler over the Jews. The Herods were Edomites. Think that made the Israelis happy? Placing a traditional enemy of theirs, over them? Not too surprising that they revolted then is it?