This is how I would argue the point....
"Did you know that the ancient Egyptians used to make paste out of dead mice and place it on infected teeth inside their mouths? It was used for toothache pain. Were they wrong to do that?
Did you know that doctors in ancient Iraq used to use sheep liver to diagnose human illness? Was that wrong?
Did you know that doctors in the 18tha nd 19th century used to cut off half of the tongue of people who stuttered in an attempt to stop their stuttering? Do you think that was wise?
You see, you can't argue that something was not wrong just because it was once accepted. Immunizations were rejected by the ORG way back when and people probably died because of that, was that God's will or his light? You can't argue that God "wanted" those people to die...
So, what's the difference with that "belief" and a "belief" where the Bible is the point in question? If the "generation" is not what was once taught but now a series of groups whose live's overlap, then the first understanding was absolutely wrong.
OR- you could simply scratch your head and say, "you are delusional, aren't you?"