From the Preface to the English edition of “The Ideas of the Holy”:
In this book I have ventured to write of that which may be called ‘non-rational’ or ‘supra-rational’ in the depths of the divine nature. I do not thereby want to promote in any way the tendency of our time towards an extravagant and fantastic ‘irrationalism’, but rather to join issue with it in its morbid form. The ‘irrational’ is to-day a favourite theme of all who are too lazy to think or too ready to evade the arduous duty of clarifying their ideas and grounding their convictions on a basis of coherent thought. This book, recognizing the profound import of the non-rational for metaphysic, makes a serious attempt to analyse all the more exactly the feeling which remains where the concept fails, and to introduce a terminology which is not any the more loose or indeterminate for having necessarily to make us of symbols.
Before I ventured upon this field of inquiry I spent many years of study upon the rational aspect of that supreme Reality we call ‘God’, and the results of my work are contained in my books Naturalism and Religion and Die Kant-Friesische Religions-Philosophie. And I feel that no one ought to concern himself with the ‘Numen ineffabile’ who has not already devoted assiduous and serious study to the ‘Ratio aeterna.’
Basically, religion cannot be understood (nor can much else) in a purely rational manner. This is not to say that it is irrational, but that it has components that transcend rationality. At that point feeling and symbol are vitally important when talking about religion. That’s why the Eucharist is so central to what Christians do, for example. It is a moment of transcendence of the rational. It’s symbol in the sense that it represents, but it can only be talked about symbolically because of its transcendental qualities. But that’s a huge debate for another time. What I appreciate about what Otto says here is that you need to think about religion supra-rationally, but that does not give you permission to talk irrationally. You don’t get off the hook whenever you claim ‘childlike faith.’ Religious experience is better than that.
Anyway, Idea of the Holy is an interesting read. I’m going through it right now in its entirety, and I’d recommend reading the classic text to anyone interested in religious theory.