This article seeks to consider the meaning of the (Christian) concept of God for Fundamental Theology. If it is usual for the (Trinitarian) concept of God to be stud ied in the context of Dogmatics, the perspective presented here is a different one. At the same time, this results in a slight broadening of and indeed alteration to certain perspectives of Fundamental Theology. In fact, the latter cannot be reduced to the study of the (rational) conditions of understanding and even of validity of a concept of God given in an absolutely dogmatic and positive form. Even if this approach, in the style of the traditional praeambula fidei, continues to be essential, it is best not to lose sight of the fact that Fundamental Theology, in being fully theological, parts straight away from the impact that is brought to bear by a (determined) concept of God revealed and transmitted through the ecclesial tradition, and hence not deducible a priori through categories that pretend to be rational. This article considers this impact from the perspective of the possibility of thinking God, of the truth of the concept of Him, of the historical origin of His constitution-revelation and of His Trinitarian content.