A collection of nine letters from the Catholic theologian Romano Guardini to his friend Fr. Josef Weiger was published in 1927. In Ludwig Mies van der Rohe’s library of several hundred items, it was a book that he used to formulate the basic principles of his own architecture. The aim of the paper is to present the content of Guardini’s book and its influence on the views and work of Mies van der Rohe. The architect’s insightful monographs, especially the book by Fritz Neumeyer, accurately document the relationship between the theologian’s statements and attempts to adapt them to the theory of architecture. The more recent analysis of these relationships presented in the article leads to the conclusion that Mies van der Rohe set himself the task of finding a pure spirit in the world of pure abstractions. His creative involvement was a form of contemplating and developing forces which, although contained in nature or technology, can also be considered transcendent.