I don't get your problem with Falwell. He is a Christian who has read the Bible. Everyone knows that when the people of Israel strayed from God tragedy befell them. They spent a lot of time as slaves. Falwell is simply warning us out of Love according to his Faith.
This does not shift blame from the Islamic attackers it is a comment on the nature of the defenders (us).
We need to take what he said and learn from it.
IMHO it sounds like Falwell jumped to conclusions by not qualifying his inference with a "maybe." Not every personal (national) misfortune is associated with a particular personal (national) sin, though some are.
If any one national sin had to be most directly blamed for this, I would say the sin of naive immigration policy.