My LOST Djinn Theory for Dummies

http://en.wikipedia.org/wiki/Jinns
Djinns are also called Jinns and Genies. They are in ancient folk-lore all around the world.
They are what we would call 'demi-gods', or 'spirits', perhaps even Angels and Demons.

Way back seasons ago, this theory about the nature of Jacob and what turns out to be Man In Black, and their fight of Good vs. Evil. I did not first think of this, but a light went off that said.."this is it..this is what they are".

So without re-posting all that is in the Wiki article about them, (link above) i will 'dumb it down',
by describing a very familiar 'Jinn' to you and how she is exactly what we're talking about.

That's right: "I Dream of Jeannie"!

1. What does Jeannie do when she goes in and out of her home..the bottle?
Turns into a column of SMOKE!!!

2. Jeannie can turn into other people or change clothes in a wink of her eye.
she is a shape-shifter, just like what we've been seeing.

3. Jeannie has an 'evil/sexy' (or in the show, a 'naughty') side; that is dark-haired and wears black clothes and gets people to be 'bad'...The Girl in Black.

4. Jeannie cannot harm her captor..."Master", she calls him. but she gets him into lots of trouble by manipulating events and other people. Good thing she loves her Master. MiB does not.

5. Jeannie has possessed other people (usually to make them do or say what she wants them to)
"jinn often harass and even possess humans"

In short, I'm now sure that is what Jacob and MiB are. Good Jacob is there to keep the evil MiB in his 'bottle", the Island. He was put there ages ago as a prison. Jacob is his jailer. He enlisted the Losties as his 'end-game' to counter MiB's using Locke's body to convince Ben to kill him.
He saw it all coming, he let Ben kill him to spring himself free of his body, in order to go to the next state, not unlike Christ allowing himself to be killed to become more powerful.

It was very clear when F-Locke said "I want to go Home."

the Losties true purpose is to prevent that..to destroy MiB once and for all. As Miss "lamp-post" said "lord help us all if you fail", implying he will destroy the earth if he gets loose.
f-Locke will of course try to convince them the opposite.