17 The Treatise on the Love of God , however, is a much more important and significant book than any of the others he published. In this work the saintly Doctor gives a veritable history of the love of God, explaining its origin and development among men, at the same time showing how divine love begins to cool and then to languish. He also outlines the methods of developing and of growing in the love of God. When necessary he even goes deeply into explanations of the most difficult problems as, for example, that of efficacious grace, predestination, and the gift of faith. This he does not do dryly but, by reason of the agile and well-stored mind which he possessed, in such a way that his discussions abound in most beautiful language and are filled with an equally desirable function. He was also accustomed to illustrate his thoughts by an almost infinite variety of metaphors, examples, and quotations taken from the most part from the Holy Scriptures, all of which gave the impression that what he wrote flowed no less from this heart and the depths of his being than from his intellect.
Source: Rerum Omnium Perturbationem (Vatican.va)