My question is this… Jesus said “I never knew you, Depart from Me ye that work iniquity.” ( Matt. 7: 21 – 23 ) When will this incident occur? Will it be at the Great White Throne Judgement? If this is the case, then these people must be in hell waiting trial and sentencing; there is no “get out of jail” card if you find yourself in hell, so why would they try to talk their way out of the situation? Would they not know that it is over for them once they are in hell?
Matt. 7:15-23 is about false prophets. I believe the phrase “on that day” which appears in Matt. 7:22, refers to a judgment day. Since Jesus tells them, “I never knew you” (Matt. 7:23) we know He was speaking of unbelievers. Therefore, there are two possibilities; the day of the person’s death and the great white throne judgment. Of the two, I prefer the day of the person’s death because as you say, once the spirit of a deceased person is assigned to hell there is no chance for a change in his or her future.
But whether this is when it happens or not, most condemned people don’t just go quietly to their destiny. Out of desperation they will often try anything, in this case even making the false claim that they were working for the Lord, to avoid what’s coming.