How does God enter philosophy? This Heideggerian questioning invites today's theologians and believers (in the movement initiated by the phenomenological debate) to reassess the question in applying it to their own tradition and to theology itself. The entry of God into theology is dealt with, in a manner which is both exemplary and paradoxical, in Augustine's treatise about the Trinity. In an exemplary manner: he «discovers», in Book V of his De Trinitate, the relation as first category to express the newness and the reality of a God who is both three and one. In a paradoxical manner: he «covers up» in Book VII his discovery in including the relation in the scheme of the substantia. The theological decision, eminently justified at the time of the first Councils, can today be brought into question and should allow us not so much object to it, but to discover afresh the possibilities which it had opened up.