I think I have this figured out... by primedforillusion

At least as much as I, personally, believe it needs to be explained...

The Island housed the 'power of creation', if you will... The power of God, if we must resort to religious terms... Call it electromagnetism, exotic matter, "that energy", whatever you want... It was a power, even though we could almost grasp its potential, that simply beyond our comprehension... It was the glue that held the universe, existence, together... It allowed reality to function as it must, and it allowed the passage of the human essence to pass from one life to the next, or perhaps from life to The Waiting Room" to nirvana, oblivion, peace... It was "life, death, rebirth"

Granted, that paragraph makes certain philosophical assumptions... But I don't believe they are too hard to believe, considering how profound some of our recent quantum science has become, to believe in the context of a television show of LOST's nature with its apparent uniting of science and faith... They are two paths that run together, not in opposite directions

Now, with regard to Jacob and the Man in Black, two things should be obvious... Even if the Island is "the house of God", God is not a personal being like you might find in The Bible; he is instead a force of nature... Or, if It has any personality at all, Its choice to is to leave humans to their own affairs, to make their own choices... So, ever since human beings first found this Island, and perhaps were instructed of its importance in a dream or a vision, in terms that he or she could understand, there has been a successions of protectors... Some of these protectors, I would wager, became Smoke Monsters - the result of a corrupted human soul coming into contact with this Divine Essence

Perhaps even there was only Mother... But I believe we can say with certainty that she was both protector and Monster, because Jacob and the Man in Black were born around the time of Christ, but the Egyptians (yes, an assumption, but one that should be obvious by this point - we don't need to be told that Egyptians lived on the Island and built the Statute - it is a given) had knowledge of a Smoke Monster judging by the hieroglyphics... In fact, given the Mayan influence of the Temple, it seems that many ancient civilizations encountered the Island, perhaps by design, and it influenced all of the world's religions

That is the beauty of all of this... This blending of science and faith

Anyway...

Jacob and the Man in Black present a unique situation for the Island, who has probably only had a protector or a protector/Smoke Monster until Jacob threw his brother down the well... Now a frightening possibility exists... The problem with Smokey leaving, from what I can grasp, is not in what would happen if he leaves... It is what he must do, or believes he must do, or perhaps simply desires, to leave that is the problem... If he is to destroy the Island, the Man in Black knows he must destroy the Source... If he destroys the Source, the glue that holds existence together will be erased... Souls will be unable to "move on" after death

A side note... What is after death? We don't know... And it doesn't matter

So Jacob, knowing his brother may succeed in killing him and wishing to show him the potential humanity, so that he might renounce his mission to destroy and leave the Island, must always bring Candidates there... He explains his own reasons why he chooses the people he does, and I need no more explanation than fate or the universe, in its own desire to remain in existence, ensures that they will come... Regardless of what metaphysical forces may be in play, all of our characters make choices - their own conscious choices - that lead to their coming to the Island... And once they are there, they are able to be made whole again, to be redeemed for the mistakes of their past and find a sense of community that they never felt anywhere else in their lives... It is the most important time of their lives, as Christian says, the time when they find out who they really are

All the Island or Jacob does is give them a nudge... Even that word may be overstating it

And Desmond does the same in The Waiting Room... Once they remember their true lives, realizing the fabrication of the 'sideways universe' - which, I might add, still has some connection to the exotic matter, if it is truly "life, death and rebirth", i.e. this final acceptance and redemption in The Waiting Room is only possible if they save the Island - our characters are able to accept that they were meant to come to the Island and complete the "work to do" that results in saving the world and maintaining existence itself

Although they are privy to only flashes as individuals, enough perhaps to allow them to make a choice and take a "leap of faith, once our characters are together, they are able to "remember" and then "let go"... They move on as a group, a family, if you will... To what? It does not matter and it is not for us, as mortals on the wrong side of the fence, to know... But we can trust that, if we one day make our own journey, we can thank the work of these individuals

My tongue is slightly in my cheek on that last bit, of course

So, that is it... I suppose that may not answer all of the smaller questions that we wish to fully understand, but that understanding of this show, which I believe should be consistent with everything we have seen, left me in awe as I was rewatching "The End"... This show took to the Island, from the Island, through time, and between life and death... It is the most epic story ever told, at least in this generation, and I truly say, without a doubt, that I could never have been more satisfied

I hope, if you read this far, you enjoyed the ride... I know I did