When did the teaching that salvation can still be obtained by people after their physical death first appear in Christianity? Since the time canon was formed, when did the teaching that salvation can still be obtained by people after their physical death first appear in Christianity?
The idea shows up very early on, dating back to apostolic times.  We see Paul talking about baptisms on behalf of the dead (1 Corinthians 15:29) as if it were a familiar, non-controversial practice, and in Peter tells how Christ went to preach to "the spirits in prison" while he was dead, (1 Peter 3:18-20) and saying that they were sinners who had been disobedient in their lives.  So it's safe to say that the concept has been around since the beginning of Christianity.