The problem is that the diagnostic system of psychiatric disorders makes explicit exeptions for religious beliefs. If you believ that Jesus was born by a virgin you are not delusional, but if you believe that your became pregnant by some godly interventin you are delutional. This is a bit illogic, I agree, but the vergin bith of Jesus is so established in the western culture so we cannot say people believing i it are delutional in a strict way.
This one is a bit harder... someone believing that Jesus returned invisibly in the year 2011 is probably delusional......but a group of poeple in an established religion stating that he returned in 1914 is probably not delusional in a strict psychiatric meaning. Even if we both, and probably most people here and everywhere find that belief just outright crazy.
Neverthess, my point about scaring people back to the kingdom hall is about how to interact with believers.......calling them delusional doensn't make them listen to us...........they just identify us as aggressie apostates.