Religio71,
James (the Greater), the brother of John, was killed by Herod (see Acts 12:2). (You may note that after James’ death, in Acts 13:2-3 Barnabas and Saul (Paul) were “separated” for the “work whereunto I have called them”, and had hands laid on them, and thereafter is when they are stated to be apostles when they are talked about in Acts. The Holy Ghost had manifest that they were the ones to be called. One was replacing James as an apostle among the twelve.)
So after Peter was later also killed, then John was the senior apostle. At the end of his mortal ministry, he was exiled and imprisoned on the isle of Patmos, from which he wrote the book of Revelation as he had visions and revelations about the end times. He was the senior apostle when he had those visions and revelations on the isle of Patmos.
John was the one who had been designated by Jesus as “that disciple” who “should not die” but would “tarry till I come.” (John 21:23) John continued to tarry on the earth, but was no longer “mortal” in the sense that his body was changed at the end of his mortal life span so that it was no longer subject to death, and that is what “tarry till I come” means, that John would be on the earth until the Second Coming of Christ. But he was no longer mortal, and a mortal person would need to have the keys of the apostleship in order to carry on the work of the apostles that Christ had ordained them to do, to carry the gospel to the ends of the earth. John was evidently the last living apostle.
Peter, James, and John were the ones who restored the keys of apostleship to Joseph Smith and Oliver Cowdery in 1829.
