This paper argues that the nature of reason has a prior and undifferentiated knowledge, purely and necessarily, of what religion is. Accordingly, whenever the reason encounters the reality of religion, based on this prior knowledge, it succeeds in recognizing this reality, and whenever reason encounters a pseudo religion, it does not accept this kind of religion and put it aside on the basis of that undifferentiated knowledge. Based on this undifferentiated knowledge, many of the definitions of religion are disregarded. Eventually, this knowledge is revealed both in theoretical and practical reason and the two are related to each other. These foundations in theoretical reason are the origin of the being, the guide of existence, and the ultimate world, and in practical reason, are the origin, the guide, and the world of morality. The principles and the bases of being and morality are the knowledge of the nature of reason that together provide a definition of religion that can be accepted by all the people and can be far from personal and individual opinions. By this definition, we can get closer to the universal knowledge of religion.