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 [IVP], for some great insight on this subject). I happen to agree with this warning.
The most dangerous plans in history were carried out by people who were very certain of both themselves and their actions. Do you recall Saul of Tarsus persecuting devout Christians? Do you recall wars carried out in the name of Jesus Christ, the man of peace? Do you recall the church putting people to death because it was so certain that it was doing God's will?
Beware of the type of certainty that is built on a false foundation. Anyone who says "I know" should always say so with a great deal of humility about both their knowing process and the conclusions they reach about what they know. This does not mean we should become relativists. And it does not mean we should be filled with deep inner doubt. It simply means we should not make a system of belief into the "truth of God."