Just say “give me call on May 22nd and we can discuss further as to why you were not raptured”. The first point would be that the idea of a rapture as they claim is completely un-biblical. “they were caught up with” does not mean, they dissipated from the face of the Earth instantly. Also, if I’m not mistaken the Bible clearly states that believers would be around when all the bad stuff happened - they would be sealed of course - thus not taking part in the “punishment” but existing in human bodies non the less.
If we should concern ourselves with this “prophecy” at all I believe we should look at it as an opportunity to share the faith through acts of love. When the rapture fails to occur you will be looking at thousands of people who, will know instantly, that they have been deceived. This will probably invoke fear in them and will cause them to question themselves and God in ways they never had done before. Let’s be ready not to say “I told you so” but rather say “You are always welcome here”. I doubt anyone who has put their faith in this prophecy will let go until the time has past. The reason being is because there will either:
A) Be a rapture
B) or Won’t be a rapture
and thus faced with the decision to:
A) Believe in the rapture
B) Disbelieve in the rapture
Thus the Combinations and their effect on May 21.
A and A → they are right - Raptured
A and B → The people risk not being raptured (Bad-huge loss)
B and A → Believed a lie (no effect)
B and B → We were right (no effect)
At this point they (in their minds) gain nothing by switching sides, but rather only risk losing taking part in the rapture.
However, since we know the rapture won’t happen, believing it will happen (option 3) means only that they are believing a lie. Just let time prove the falseness of the prophecy, saves us time which we can use to prepare further to defend our faith against more deceiving doctrines that actually appeal to logic. When all these people “wake up” to what they have been believing, blindly and falsely all this time, such defense will be important.