Why is 2 Corinthians 5:15-16 not found in Matthew, Mark, Luke, and John?
Paul's words in 2 Corinthians 5:15–16 are absent from Matthew, Mark, Luke, and John because the doctrine they express is based on Christ's death, burial, and resurrection, which had not yet happened during His earthly ministry. The four Gospels could not teach a doctrine built on events still future.
Les explains that verse 15 says Christ died for all so that those who live should not live to themselves but unto Him who died and rose again. He ties this to grace not being license: forgiveness, justification, redemption, and reconciliation bring a love requirement and debt to please Him. Then verse 16 says Paul writes, henceforth know no man after the flesh; though they had known Christ after the flesh, now they know Him no more. Les says this refers to Christ's earthly ministry. He asks whether any of this was taught in Matthew, Mark, Luke, and John, and answers, not a word, because it is all based on His death, burial, and resurrection, which had not happened yet. They could not teach doctrine based on those future events, so this cannot be found in the four Gospels. Les adds that Old Testament men were justified, but not on the same basis as under grace, and Paul does not look back at Christ's earthly ministry for these doctrines. Paul likely knew Christ after the flesh as a contemporary, and Saul of Tarsus knew about Jesus, though not by personal contact so far as Scripture says.