With my help, you can learn to join with the Force, yet retain consciousness. You can join your light to it forever. Perhaps, in time, even your physical self.
"Eternal life..."
The utlimate goal of the Sith, yet the can enver achieve it; it comes only by the release of self, not the exultation of self. It comes through compassion, not greed. Love is the answer to the darkness. - exchange between Qui-Gon Jinn's spirit and Yoda on Polis Massa, Revenge of the Sith novelization

Ever since fans saw Darth Vader make his final stroke and Obi-Wan Kenobi's robes crumpled, empty, to the deck of the Death Star and moments later his voice is heard telling young Luke Skywalker to run the ability of the Jedi to appear after their aparent death has been a mystery. To add further to the mystery years later Yoda's body disappears before Luke's very eyes on Dagobah, and after passing away the now redeemed and atoned Anakin Skywalker is seen standing next to his old masters in the forests of Endor. It was first assumed that all Jedi disappeared and came back in a spectral form until The Phantom Menace came along and Qui-Gon Jinn didn't disappear upon his death. His body, instead, was cremated.

Yet years later(10 in universe, 3 in the real world) Qui-Gon is heard from again. His voice is heard saying "Anakin, no!" simultaneously with Anakin's slaughter of the Tusken tribe in Attack of the Clones. It then is revealed in the closing moments of Revenge of the Sith that Qui-Gon Jinn had managed what no Jedi of his time had... he merged with the Force and retained his consciousness and ability to interact with the living. He claimed to have learned it from a shaman of the Whills and sought to teach how to do this to Yoda and Obi-Wan Kenobi. As can be seen from the quote above, this was accomplished through total surrender of the self to the Force and through compassionate living.

This particular facet of Star Wars is seen in religions throughout our own world. There are qi gung schools which teach to cultivate one's own life force for immortality in Taoism. There are similar concepts in yogic schools. My primary concern here is going to be with Islamic and Sufi notions of "fana wa baqa" or annihilation and subsistence, and "shahadat" or witnessing/martyrdom.

"Die before you die." -Prophet Muhammad, peace be upon him and his Household

First and foremost I want to say just how unqualified I am to really speak on this topic, so I'm really only going to briefly go over what little bit I do know. All is taken from books rather than actual experience.

So, let us begin. Firstly, what does it mean to die before death and why should we? This is not a fatalistic statement, rather it is an affirmation of the limitations of the life of this world and the forms percieved there-in. In Islamic theology it is said that upon death man truly awakens to Reality. Therefore, Muhammad counseled his followers to die before their physical death that they might awaken to the Reality(al-Haqq) of God.

A person who has done this is a witness(shahid) to Absolute Truth and Reality. He has achieved a state of annihilation(fana) of all conceptualizations of falsehood and unreality. In short, he is possessed of no illusions. Be they of what this world really is, or of his own ego and ultimate unreality in the Face of God. The heart has, at this point, been polished of all tarnish and shines the Light of God unobstructed into the world. The person that has attained this station has joined the essence of his own being, his own light, to the Light of God and thus finds his subsistence(baqa) with his Lord far beyond this world into Eternity.

Qui-Gon Jinn, in telling Yoda that he must surrender his "self" in order to merge his inner light with the Light of the Force is telling him exactly this... "you must die to your self before your physical death in order to be one with the Force." He must do this through love, through compassion. We are not talking here about the petty love that Anakin Skywalker expressed for Padme... but rather the kind of selfless love that puts the Beloved first and foremost... that wishes not to possess, but merely be in the Presence of the Beloved. For the Jedi the Beloved would be the infinite mystery that is the Force, and through the Force they would express their compassion for all living beings. Even their enemies.

Qui-Gon, presumably, instructs Yoda and Obi-Wan for the next 20 years on how to achieve this. We don't know the precise method of instruction or method of attainment... but it must be through deep meditation, perhaps some cultivation and refinement of internal energy and definitely long hours of meditation that surrenders the illusory self. Which actually brings me to an interesting facet of Sufism, instruction from the intermediary realm(barzakh) between this world and that of the spirits.

Uwaysi Transmission

There is an interesting element to Sufism that has its roots in the stories of the life of Muhammad and a man named Uways al-Qarani. Muhammad lived in the Hijaz region of northwestern Arabia while Uways lived in the Yemen. There had been no physical or written contact between the two men, and yet Muhammad told his son-in-law Ali and another companion to go to Yemen specifically to meet this Uways al-Qarani. When they found him he had knocked out all his teeth, and both questioned him why he did this. He told them that he'd known Muhammad had been injured in battle and one of his teeth had been knocked out so he knocked all of his out not knowing which one it was precisely. Uways and Muhammad has been in contact in the intermediary realm(barzakh) and Uways has learned all about Islam from him there. To this day there are Sufi orders that trace their lineages back to a Shaykh(elder, teacher, master) who had been instructed by another Shaykh either from a distance in the subtle realm of the barzakh, or from that world after the instructor had previously passed from the physical realm.

Yoda and Obi-Wan's learning of this ability to merge with the Force from Qui-Gon Jinn was exactly this sort of transmission of knowledge. Later, Luke Skywalker recieves similar training from Obi-Wan. We see hints of this both with Obi-Wan's advice to Luke as he flies down the Death Star trench, and upon his telling him to visit Yoda and even conversations that they had. In Splinter of the Mind's Eye by Alan Dean Foster(the first installment into the Expanded Universe) there is a point where Obi-Wan literally guides Luke in his first duel with Darth Vader from the beyond. Recently in Timothy Zahn's Allegiance there are apparently scenes with old Ben instructing Luke to a limited degree.

So, if this ability to merge with the Force is something of instruction and cultivation of selflessness... how did Anakin Skywalker manage to become one with the Force in Return of the Jedi?

"Do not say that those who are killed in the way of God are dead, for indeed they are alive, even though you are not aware." - Al-Qur'an: A contemporary translation by Ahmed Ali

Aside from the official canonical answer regarding Anakin's subsistence in the hereafter, that Obi-Wan and Yoda managed to help Anakin retain his identity in the moments before his physical death, there is another plausible explanation in light of Islamic teachings. There is the concept of shahadat in Islam, it directly translates to "witnessing" or "to see" but in technical usage the closest translation to a similar notion in English is "martyrdom." Far different that the usage by Muslim extremists and their apparent death cults, martyrdom in Islam is not restricted to dying on the battlefield or execution for what one believes. Though this is one element.

Rather, the concept of martyrdom in Islam in intimately linked with "jihad" meaning not "holy war" but "striving" specifically striving in the way of God. This could take the form of battle against tyranny and oppression, but it also takes the form of the inner battle against being dominated by the animal instincts and desires and against the suggestions of the Shaytan. It is the struggle between the Light and darkness played out within the human heart. Anyone who is a true striver(mujahid) in the path of God becomes a martyr(shahid) upon death...whether that be of natural causes or upon the field of battle against the forces of tyranny and darkness.

All three of our Jedi spirits are prime examples of this martyrdom(shahadat) in the Original Trilogy, and Qui-Gon Jinn is a prime example in the Prequel Trilogy. Though you may ask why other Jedi who died in the struggle of light against darkness in the Clone Wars or the Jedi Purge did not achieve the same status. Well, shahadat is a bit more nuanced than that. You see, the word means "witnessing" but what is being witnessed? Truth. Reality. In the entire saga no other Jedi had truly witnessed by the four which have retained their identities. All sacrificed their lives that the truth of the Light might continue on.

Obi-Wan Kenobi knew that Luke needed to become a Jedi Knight in order for the Light to prevail, he also knew that Luke wouldn't get off the Death Star(and neither would Leia or Artoo for that matter) if they waited for him. He laid down his life so they could escape, knowing that his true life was with the Force and having his subsistence with the Force... he could guide Luke from the Netherworld. He would also be an inspiration to Luke in that moment. Yoda was teaching the first of a new Jedi Order and he likewise knew that the Force was everything and when he passed on he died in the cause of Light.

Anakin Skywalker, though, died under different circumstances and is perhaps the greatest of all examples of shahadat in the saga. Mere moments after he was serving the tyrannical Darth Sidious he turned back to the Light and hurled the Dark Lord into the Death Star's abyss. As he watched his son writhe in pain from the lightning hurtling from the fingertips of his dark master he realized the error of his ways... he witnessed the truth of what was at stake, what he'd become, who he'd been serving and who his son was... and more importantly, the Will of the Force. He saw the Truth, and he chose to sacrifice his life for it.

Now, we know that Obi-Wan and Yoda helped Anakin across... but how could they teach him what took them both 20 years to learn in moments? Perhaps they didn't teach him at all... Anakin had already sacrificed himself... both his physical life and his ego... for the Light, for the Truth... but was this enough to grant him subsistence from the Force? Shahadat carries with it an interesting note... martyrs are granted intercession for 70 people on the Day of Judgement. Yoda and Obi-Wan interceded for Anakin and helped him make the full transition to subsistence with the Force.