Faith is clearly not enough for many people. They crave hard evidence, scientific proof. They long for the scientific seal of approval, but are unwilling to put up with the rigorous standards of evidence that impart credibility to that seal.Which is interesting. So many people have faith-based beliefs that they couch in the language of science without ever understanding the nature of science. They claim "The science is settled" without realizing that that is in itself a faith-based claim. That science is never settled.
In fact Sagan refers to this elsewhere in the book.
We will always be mired in error. The most each generation can hope for is to reduce the error bars a little, and to add to the body of data to which error bars apply. The error bar is a pervasive, visible self-assessment of the reliability of our knowledge.