V'ger - an allegory
As the movie reaches its climax, it is revealed that V'ger is the old Voyager 6 probe, programmed to gather data about the Universe and send the data back to Earth. The battered space probe was found by artificially intelligent life forms. After looking at the original programming on the probe, the machines interpreted its mission in the broadest sense possible - learn all that is learnable and transmit the knowledge back to the Creator. The machines constructed an enormous spaceship to enable V'ger to fulfill its mission. As a side effect, the enormous amount of knowledge in its data banks leads V'ger to develop consciousness.
As its journey nears its end, V'ger has a crisis of faith, wanting to see the Creator and learn if there is nothing more to its existence than seeking information and bringing it back to the Creator. In this respect, it is an allegory of the search for God by humankind. However, when it first comes into contact with the Enterprise, V'ger refuses to accept humans as "true life forms", to which First Officer Willard Decker replies: "We all create God in our own image."
Spock recognizes that despite its age and experience, V'ger is immature, like a child. He recommends to Kirk that it be treated as such, despite its awesome power. After V'ger states it will "remove the infestation on the Creator's planet"—in other words, humankind—by destroying its surface, Kirk plays on its instinctive want and need for its Creator with this gambit: he tells it he knows why the Creator does not respond to V'ger's repeated calls. He tells the assimilated Lt. Ilia, however, that he will only disclose this information to V'ger directly.
Kirk, Spock, McCoy, the Ilia probe, and Decker are permitted to enter V'ger's central core, where they find the ancient Voyager 6 probe and realize they can trigger its radio transmitter, thereby proving that humankind is V'ger's 'Creator'. V'ger, however, wants to join with its Creator in order to experience levels of understanding and being beyond its cold, logical mind. Decker volunteers, and the two fuse into one as V'ger enters into a higher plane of existence.