Concerning the statement in II Thess. 2:1-3) that the day of our gathering to Him will not occur until the man of sin is revealed: I don't understand why you think that statement implies that the gathering of the elect in Matt 24 was fulfilled during NT times. To my mind both events will occur in the future. Or do you interpet "the man of sin" to be someone who lived during the first century—and if so, who?
Irenæus (120-202 A.D.) believed the Man of Sin, or the Beast, or the Antichrist, whose number is 666, would come in the future from the time he wrote "Against Heresies". (See Book 5 Chapter 30).
There is no clothing in a robe of righteousness, that poorest of legal cobwebs spun by spiritual spiders. — George MacDonald, Unspoken Sermons II, The Last Farthing
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