The parousia refers to the return of Jesus Christ.  The New Testament expected a return in the first century but obvious that did not happen.  Now the Church prays maranatha, Our Lord Come!

The first Christian writer Paul, or his editor, predicted that there would be a mass falling away from the gospel when the antichrist comes.

This figure was expected before Christ returns.  So he warns not to listen to anybody saying that Jesus was about to come back or already had (2 Thessalonians 2). Yet the synoptic gospels, that is Mark, Matthew and Luke which are the ones that have the most in common, with John the fourth gospel being the odd one out, strongly emphasised that Jesus could return any minute which was why people were counselled to be ready at all times. Even 1 Thessalonians 5 stresses that Jesus’ coming will be sudden and it will be like a thief in the night and there will be peace and then instant destruction before anybody has a chance to escape. Christians say that there will be great calamities after this peace and the calamities will be sudden and destroy and be over fast and then Jesus will manifest himself. But this is wrong because the thief comes at night when all is quiet and peaceful. The destruction will accompany the appearance. Everywhere else the picture is of a destruction that is not all that sudden so there should be time to escape. The contradiction plus the evidence for Pauline authorship suggests that somebody revised this letter after Paul wrote it. It was so easy for anybody to do something like that for first there would have been one copy and then somebody would write another copy and somebody would copy that copy and so on especially when there is no evidence that Paul took measures to safeguard the authenticity and preservation of the text or even cared if the letters were preserved.

2 Thessalonians warns against anybody forging letters in Paul’s name but that only means letters that said that Jesus was coming back on such a such a date or is already here. The obvious lie about Jesus would prove that the letter is a fake. It does not imply that they made sure that the text of existing letters would escape interference like the Jews did with their scriptures. Paul would not have been so vague at times if they had because that would be an invitation to people to interfere. Christians say there is no evidence of interference. Even if that’s true, who is going to alter a document and make it obvious and obvious all the time?