Paul addresses the likely reason for this second letter, a misapprehension of the second coming. Some had been caught up in the belief that the second coming had happened or was about to happen soon. This idea may have come from a false letter of Paul.
The end times will be accompanied by a man who is against God’s law but who exalts himself over God, and this was something that the people would have understood. Most men, including Christians will fall away but those who endure to the end will be saved.