A true believer has total confidence based on faith...
not a certain level. Do some believers have doubts? Of course, but this is considered a weakness that must be put aside.
Changing scientific beliefs are part of the search for truth. As ER commented truth is a moving target that must be constantly debated and refined.
Science and philosophy is based on a search for ultimate truth, a theory of everything. We may never find it but we will move closer while those who rely on faith are frozen in time.