Paul had to remind these believers they were not in the Day of the Lord because 2 things had to occur first. There would be The Great Apostasy and when this event happened world wide then the man of lawlessness would be revealed. Today we are seeing signs pointing towards a day when the whole world we rebel again God and oppose him, His Truth and His people. This is happening in 2018 in most western countries today where legislation has been passed to remove free speech and freedom of religion. All pointing to the future time where a man would take control of the world and its systems and declare himself to be God and demand worship.