This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The book explains that rationalistic discussion of religious beliefs is not promising when it comes to many forms of religious disagreements and explores understandings of religious beliefs according to which they are beliefs of a scientific type, based on evidence or on scientific reasoning. It also discusses that the relativist suggests that the distinction between true and false should be given up in a situation where it proves impossible to reach an agreement concerning the truth or falsity of the issue in question. The book examines by criticizing the demand for universality, according to which every statement has to be argued for by means of conclusive arguments which show that the statement in question follows from other statements which have already been proven in this way or are self-evident.