"What is God?" asked Thomas Aquinas as a child. Contemplation of the mystery of God, in the light of reason enlightened by faith, is the heart of the theological enterprise. It is based on the Revelation that God has made of himself in the history of salvation and, in order to foster the understanding of faith, it assumes the best of metaphysical reflection, since the saving God revealed in Jesus Christ is not another God than the Creator of heaven and earth, the subsisting Being, the First Cause of all being. The One God deals with a first "part" of the mystery of God: God contemplated in the unity of the essence common to the three persons of the Holy Trinity. It explores the ways of our knowledge (section 1) and presents the theological question of his existence (section 2). It then sets out the perfections of God's very being (simplicity, goodness, infinity, eternity, etc.) (section 3) and, after a critical review of the value of theoretical knowledge and language (section 4), presents the principles of his action (knowledge, will, omnipotence) (section 5). The One God is a continuation of the treatises De Deo ut uno, which, in the form of a systematic, historical and doctrinal commentary on questions 2-26 of the Prima pars of the Summa Theologiae of Saint Thomas Aquinas, have succeeded one another throughout the history of the Thomist tradition. Its aim is to update a theology treatise that is unjustly neglected today and to offer teachers and students a solid and informed manual on this fundamental subject.