The Fall reflects the existentialist claim that there is no objective truth. What we think of as fact is a set of beliefs. In fact, that novel’s main conflict arises from those beliefs being shaken by the narrator. This narrator then concludes, almost jovially, that truth is overrated. For one, it’s boring. It’s also not that useful. According to the narrator, "truth," as we think of it, isn’t always as illuminating as lies.
Jean-Baptiste’s purpose in confessing his sins to you is to garner understanding. He fears nothing more than isolation.