I wish people would stop assuming that LOST is just doing an Adam and Eve thing here. "Across the Sea" pretty much threw all of that out when they proved that 'Adam and Eve' were the Original Island Lady and Nameless Guy.
It could be they were borrowing from Jacob/Esau, but you could probably go through any other texts, myths, stories, histories, etc and find stories about twin brothers who are left somewhere and raised by somebody not their real mom. Um, like Romulus and the other guy raised by a She-Wolf. :P
My theory is that the OIL is an immortal who is supposed to look over the island and keep the Island Power from escaping the island or falling into the wrong hands.
She has been at it for a long time and wanted a successor. She chose to take the twin boys, because they were innocent and untouched by the corruption of mankind.
MIB had a special gift or two and always was the one she favored. He was the one she wanted as a successor, and Jacob saw this, even though he was the slower of the two kids.
He wanted to stay on the island and he wanted his brother to stay too. And whether he intended or not, he found a way to make his brother stay.
His actions and the 'offering' to the cave meant that his brother merged with the island power. <- That means his spirit will be forever chained to the island, as long as there is a protector keeping it there.
Jacob is now looking for a replacement - and he has made a game out of it. You remember the MIB told him that since he found that little dice game, he got to invent the rules.
The game as it stands between him and MIB will continue until a replacement is singled out or all of them are killed. He needs all of them to die so he can be free of the island.
Meanwhile, he is being haunted (remember, he can see the dead) by Jacob, etc...
At any rate.... people can put their bibles away now and enjoy the show. ;)