When Obi-Wan fights Darth Vader in the New Hope, he says:
You can't win, Vader. If you strike me down, I shall become more powerful than you can possibly imagine.
What does he mean by that? Is it true? If it is true, why does he tell Vader?
He is referring to the fact that he will become one with The Force - a technique only a handful of jedi have learned.
As a 'spirit' he will continue to guide Luke, and thus become somewhat omniscient - subsequently becoming more powerful.
You might say as an older man, he had become physically weak, but once bonded with the Force he becomes greater than the sum of his parts.
Obi Wan learned a specific force technique from Qui-gon Jin that allows him, after death to retain communication with the living via the force. This partially explains Kenobi's comment.
What Obi Wan is most likely referring to is the fact that if Vader kills him it will drive Luke to learn the force and take on the Empire. Obi Wan believes that since Anakin was not the chosen one, the one who would bring balance to the force, (although arguably either side winning results in unbalance) that Luke is in fact that person. Kenobi is aware of the fact that Luke will witness Vader killing him while he is clearly in a pose of surrender, and he believes this will motivate Luke to become a Jedi Master and a match for Vader.
So Obi's comment foreshadows at least two major plot developments - his post-death ability to tutor Luke in the ways of the force, and Luke's increased interest in the destruction of the Empire/Vader.
You need to understand clearly one crucial point.
Only a Jedi who adheres to the 'good' side becomes part of The Force in the event of his death.
A Jedi adherent of the dark side who perishes is truly dead; he cannot survive as a part of The Force. Hence Vader, having adopted the dark side, had no knowledge that a Jedi was capable of surviving beyond death. This is why Kenobi warns him that he has a surprise coming if he strikes Kenobi down.
We learn later, in 'Revenge of the Jedi', that the Emperor has imbued Vader with awesome powers, by courtesy of the Dark Side. It gives tremendous power in life, but there is no existence after death. Vader has learned about the Dark Side from the Emperor, so they are both ignorant of the truth about The Force.
Ultimately, Vader saves Luke from the Emperor, turning against the Dark Side, thereby redeeming himself; hence Anakin is accepted into The Force, and Luke can see him - or his 'ghost' or spirit - become a part of it.
It is interesting to note, from Leia's reactions in that final scene, that only Luke can see Kenobi, Yoda and Anakin. Only another Jedi has the power to communicate with one who has passed into The Force. Others, such as Leia, cannot see or communicate with them.
Its a reference to the power of selfless action for the greater good. Martin Luther King and Ghaundi both acted in similar fashion. Their deaths were a catalyst of greater social change.
The original act of selflessness for the salvation of humanity was that of Jesus Christ. Obiwan's death (Oh be One is a play on words refrencing the union of Christ the Son and God the Father) is an homage to Christ's death and resurrection. Jesus appeared multiple times to his disciples after having risen much like Obiwan appears to LUKE as a spirit several times through out the trilogy.
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