The most dangerous directions taken in history were undertaken by people who had a high level of certitude for both their views and choices. This is true of dictators and tyrants, as well as religious zealots, including some Christians.
There are several ways that we seek to justify what we believe. One is by using what we think is a reliable process. Another way is to show that the world is a more coherent place because of what I believe than it is without the truths that I believe. A third way, and the one I often question in my teaching, is to argue that what I believe is inferred from previous beliefs about how things seem to be right now. This is the approach of classic foundationalism. Foundationalism seeks to build one’s beliefs upon a basic stucture that is foundational for everything else. It understands that there are indubitable beliefs from which further propositions can be inferred to produce a superstructure of known truths. Traditionally foundationalism forms beliefs about our sensory experience and then these beliefs are used to provide the basis for a superstructure of faith that is built upon the established foundation. But are these "beliefs" that make up this foundation infallible as is often claimed? That is always the sixty-four thousand dollar question.
In philosophy there are four sources of knowledge (traditionally speaking): sensation, memory, introspection and reason. When we make reason central, and in a way that leads us to believe that we know "the truth" as a complete system of humanly stated ideas, danger always lurks. For this reason many Christian philosophers and theologians are suggesting, now more than ever, that there are several types of certitude that have become graven ideologies (cf. Bruce Benson’s book, Graven Ideologies