Lost's characters and story frequently referenced the afterlife. The theme of rebirth recurred throughout the series, first metaphorically then increasingly literally, culminating in the show's conclusion, which showed the characters after death.
The Island as afterlife
Early seasons portrayed life on the Island as the characters' experiences following a metaphorical death. As Jack told Kate in "Tabula Rasa", "Three days ago, we all died." Many characters reinvented themselves on the island, redeeming themselves for old mistakes. Jacob eventually revealed that he brought people to the island to specifically give them a blank slate. ("Ab Aeterno")
Some characters believed the island was their literal afterlife. The Man in Black convinced Richard that the island was hell, and Richard reverted to this belief years later, even though he'd left the island numerous times. Anthony Cooper also thought the island was hell. ("Ab Aeterno") ("The Brig")
A pervasive theory suggested that the original crash killed the main characters. The plane's discovery in season 3 and the 324 dead bodies aboard fueled this belief. An early episode script featured Kate jokingly referencing this theory to Sayid's annoyance, and the producers repeatedly dismissed it in interviews..
Heaven & hell
Characters also referenced heaven and hell separate from the Island. Charlie tried to baptize Aaron, and later Claire asked Eko to baptize her and Aaron together, so they could reunite after death in heaven. A boy Eko knew once beat a dog to death and feared it awaited him in hell. ("Fire + Water") ("Three Minutes")
According to Isabella, the Man in Black's escape from the Island would send them "all" to hell. It is not clear how literally or figuratively she meant this. ("Ab Aeterno") The script for the finale referred to the Heart of the Island, with the Light off, as the "cavern of hell," where things have "literally gone to hell."
Off-island, Hurley believed Jack's perfect life - in love with Kate, raising a child - meant that they'd died and gone to heaven. He was wrong, but they later experienced an afterlife that was, for some, too good to be true. ("Something Nice Back Home")
The dead sometimes return, visible to specific people.
Charlie, Eko and Ana Lucia visited Hurley, who had the power of mediumship. Claudia appeared to the young Man in Black, who was also "special". But according to Hurley, you need no special powers to see the dead - non-mediums can too, but it "takes them a while". ("Ab Aeterno") Richard saw his wife's ghost, and she even kissed him. Charlie's ghost told Hurley someone would visit Jack, and Jack soon after saw his father. Many of these ghosts also appeared in flash sideways.
Miles was also a "man who speaks to the dead", but though he was called a "ghost whisperer" and appeared to exorcise a house, he neither spoke to ghosts nor heard whispers. Rather than communicate with those who'd entered the afterlife, he merely read the their last thoughts from their dead bodies. ("Some Like It Hoth")
Locke and Eko repeatedly dreamed of those whose dead bodies lay on the island. Ana Lucia, Boone, Horace and Yemi appeared to guide them in dreams after death. The show did not state whether these were people communicating from the afterlife, manifestations of another kind or simply dreams. Locke and Eko never saw actual ghosts while awake. Each did, however, while awake, see the Man in Black take the form of the dead.
When some people die on the island but cannot move on, they remain, whispering. Michael, for instance, who died some miles offshore, remained on the island for years after his death. Whisperers can eventually leave. Hurley recruited Walt to help him free his father. ("Everybody Loves Hugo") ("The New Man in Charge")
Several dead characters' bodies appeared alive on the island. Christian Shephard appeared after his body went missing. So did Yemi. A podcast called both these characters and Kate's horse "undead". Alex appeared materially present after death, and she physically moved her father Ben. These appearances were not the characters' lives after death. They were manifestations of the Man in Black. ("The Cost of Living") ("Dead Is Dead") ("The Last Recruit")
Some characters discussed the concept of reincarnation, which is the transfer of the soul into a new body after death. One of Locke's foster mothers believed his sister reincarnated as their dog. Sawyer seemed to fear Frank Duckett had returned as a boar. Kate once spoke to Sawyer as though he were Wayne reincarnated. ("...In Translation") ("What Kate Did")
The word's "Canton Ranier" on Ben's van formed an anagram of "reincarnation". The van contained Locke's body, which the Man in Black would later use as a form. Several characters, most often Desmond, referred to seeing one another in another life.
Jacob followed a unique path after death. Besides coming back as an adult ghost, he reanimated himself as his boyhood self. He then used his ashes to temporarily recreate his adult body allowing him to pass on the office of Protector to a successor. ("LA X, Part 1") ("The Substitute") ("Across the Sea")
The writers initially claimed Lost would feature no actual resurrections. "When a character dies on the show," said Damon Lindelof in a 2006 podcast, "they’re dead. The only time you’ll see them again is in someone else’s flashback." Jacob, though he appeared to heal Locke in "The Incident, Part 1", claimed he could not resurrect the dead. ("Ab Aeterno") Richard and Ben said that despite all they'd seen on the island, nothing can undo death. "You don't get to come back from that," said Ben, "not even here." ("Dead Is Dead")
Despite this, multiple characters returned to life after dying. A doctor pronounced Charlotte Malkin dead, but she awoke on the coroner's table. Though her own father dismissed this as a doctor's error, Charlotte revealed she had traveled "between places" after dying and met Eko's dead brother Yemi there. ("?") Sayid also returned to life hours after drowning. He did not reveal what he experienced after dying; the spring that drowned him in fact normally erases memories. He credited the Man in Black for his resurrection. ("LA X, Part 2") ("Whatever Happened, Happened") ("The Last Recruit")
The main characters created a metaphysical realm to find each other after their deaths. The season 6 flashes portrayed how the characters in this realm imagined different lives but eventually met one another and moved on together. Not all main characters shared the flash sideways. Walt returned to the Island planning to help his father, who'd been trapped there, whispering. Neither were seen in the flash sideways. Eko saw a vision of his brother as he died and never appeared in the other characters' afterlife. ("The New Man in Charge") ("The Cost of Living")
When characters were ready to completely let go of their previous lives, they moved on to a final stage. This involved joining with a bright light -- possibly the Light. Lost gave no details on what happened next. When asked where they were going, Christian simply said, "Let's go find out."
"How do you corral a thing that is moving outward in all directions?" asked Michael Emerson, referring to wrapping a narrative that had spanned outward into the past and future. "You bring it back to its center... a more spiritual ending was the way to go... It was -- the afterlife. Life and death."