The dawn of the new image of science, since Galileo, has called into question the Aristotelian concept of science and, thus, the old claim of the scientific profile of theology. This work deals with the idea that theology is not a science. Using Karl Popper's critical rationalism, it searches for explanations as to why it is difficult, if not impossible, for (Catholic) theology to be (regarded as) a science; and why it is still possible to speak of rationality in the context of Catholic theology