The discussion in the preface of doubt and faith are meant as a direct critique of the Hegelian "system" that dominated the philosophy of the day. For Hegel, faith is lower than reason since it is immediate and requires no reflection at all. Faith is then something that we must move beyond, according to Hegel, if we are to understand the world correctly. Similarly, doubt is associated with Descartes by the Hegelians, and is considered an earlier stage in the dialectical process that leads to the Hegelian system. This doubt is a starting point that must also be moved beyond.
Johannes contrasts the easy intellectualizing of the Hegelians with the dedication required by true doubters and believers. The ancient skeptics, for instance, maintained that since nothing is certain, they should suspend judgment about everything. Learning to suspend judgment took a lifetime of dedicated asceticism, however, since we are naturally accustomed to passing judgments on the things we sense and think about. Similarly, the faith required of Abraham was not a simple matter to be had and then moved beyond. Abraham's faith had to be so strong that he would unquestioningly sacrifice his only son to God.
The difference between these people and the Hegelians can be understood as the distinction between knowing with one's mind and knowing with one's heart. We might draw the analogy to our consciousness of death. We all know that we will die, but young people tend only to know this with our minds: we know it as a fact, and we would never deny it. However, only old people, or those who have had a near-death experience, might know with their heart that they are going to die. That is, they are conscious of their mortality in a way that informs their daily actions and their attitude toward life.
Johannes sees this contrast as one between reflection and passion. The Hegelians may think they have reasoned through faith and doubt in the same way that think their system allows them to reason through everything else. However, faith and doubt, unlike science or logic, are meaningless and void without passion. We cannot allow Descartes to do our doubting for us: in order to appreciate what it means to doubt, we need to go through the process ourselves. Anything less will lead to cheap theorizing that doesn't understand the objects of its theories. By rationalizing doubt and faith, by rendering them as parts of a system built fully by reflection, Hegelians have destroyed doubt and faith, have rendered them impotent. Kierkegaard is a staunch opponent of the Danish church of his day, feeling that faith has been reduced to an aesthetic level which cheapens the true value of religious passion.