E. L. Mascall, in his introduction to the 1966 edition of He Who Is, writes that "one of its main purposes" is to "to argue that the real function of the famous Five Ways of St. Thomas Aquinas was not to provide five independent arguments for the existence of God from finite beings, but to manifest by five different expositions the character of finite beings as radically dependent on a transcendent, self-existent infinite being." Subjects discussed along the way include creation, mysticism, the relation between reason and revelation, the doctrine of analogy and the non-Christian religions; and an attempt is made to find the true status of religious experience as an element in rational theism.

This Angelico Press edition contains a new foreword by The Rev. Dr. Robert MacSwain, who writes that "I can think of at least three reasons why He Who Is should still be read by a new generation of theological students. First, despite its age, it remains a substantial contribution to the study of Thomism. Second, its primary themes of the existence and nature of God are of perennial and vital interest. Third, it exemplifies the ecumenical endeavor that was at the heart of Eric Mascall's life and work, namely, that of an Anglican theologian seeking to learn not only from members of his own communion but from Roman Catholic and Protestant thinkers and from the Orthodox tradition as well."