"John is going to be a proxy. A substitute."

During their conversation, Eloise Hawking mentions to Jack the need of a proxy for Ajira Flight 316. As implied by Eloise, a proxy is much like a "substitute" for another person. As the Oceanic 6 must recreate the circumstances that led them to Oceanic Flight 815, the dead Locke's coffin is going to replace Christian's, although Locke must carry an object that belonged to Christian. Jack later receives his father's shoes from his grandfather, and puts them on Locke's feet. ("316")

As a result of failing to find a proxy for enough events of Flight 815, some passengers of Flight 316 were teleported back into 1977, while the rest remained in current day. The two flights were sufficiently similar for the plane to enter the window the Island existed in and crash, but not sufficient enough for all of the passengers to emerge in the same place. ("Lost: A Journey in Time")

Similarities between Oceanic Flight 815 and Ajira Flight 316

Flight 815 Flight 316 ("316")
1x05 Christian'sDeadBody.jpg
Jack loaded Christian's body on the plane, wearing Jack's tennis shoes. ("White Rabbit")
5x06 Locke'sDeadBody.jpg
Jack loaded Locke's body on the plane wearing Christian's shoes.
1x05 ShoutingAtChrissy.jpg
Jack had a heated discussion with a flight attendant at the airport about his father's corpse. ("White Rabbit")
Hurley had a heated discussion with a flight attendant at the airport about flight passengers.
1x24 Hurley'sComic.jpg
Hurley read a Spanish comic book. ("Exodus, Part 2")
5x06 HurleyAndHisComicBook.jpg
Hurley read another Spanish comic book.
1x24 Charlie'sGuitarOnBoard.jpg
Charlie carried his guitar. ("Exodus, Part 2")
5x06 Hurley'sGuitarOnBoard.jpg
Hurley carried a guitar case.
1x02 KateInCustody.jpg
Kate was escorted by Edward Mars. ("Pilot, Part 2")
Sayid was escorted by Ilana.
1x01 RoseAndTheRing.jpg
Rose was holding on to Bernard's wedding ring.
("Pilot, Part 1")  ("Walkabout")
5x06 SunAndTheRing.jpg
Sun was holding on to Jin's wedding ring.
1x24 HurleyIsLate.jpg
Hurley almost missed the flight. ("Exodus, Part 2")
5x06 BenIsLate.jpg
Ben almost missed the flight.
1x01 SethThePilot.jpg
Seth Norris replaced Frank Lapidus as the pilot for Flight 815.
("Pilot, Part 1")  ("Confirmed Dead")
Frank Lapidus was the pilot of Flight 316.