Rambam's "Perfect Faith"

Rambam says in his 10'th Principal: "I believe with perfect faith that the entire Torah that we now have is that which was given to Moses." Rambam considers any Jew who does not agree with this principal (and the other 12 principals) to be a heretic who will not be rewarded with an afterlife.

However, we have strong, objective evidence from textual criticism, archeology, and other sciences, that the Torah was written by various authors over a long time span. The clinical definition of delusion according to University of Michigan Medical Center (which offers treatment for psychiatric patients) is: "A firm, fixed idea not amenable to rational explanation and maintained despite objective evidence to the contrary..."

How is it possible to meet Rambam's criteria while avoiding delusion at the same time? Or is the afterlife only for the delusional or the ignorant (of the contradictory evidence)?