Trinity, ontological difference, substance, subsistence 

The main question in trinitarian theology is how God can exist both in one essence and in three persons. A highly sophisticated approach to explain this is taken by the bishop of Poitiers, Gilbert - and he has been criticized by Bernhard of Clairvaux. This paper tries to outline the basic ideas of Gilbert and shows that Bernhard misunderstood this position in defending the ''orthodox'' dogmatic belief. Gilbert used the difference between the substance and its substantiality to follow the problem of whether God is God through himself or through an entity distinct ( or at least distinguishable). And if this Godness would be the constitutive moment of Gods essence, is it then God itself or something else. In Bernhards critics it becomes obvious that he misses the decisive point: Gilbert does not invent some fourth person or entity in God but endeavors the ontologically formal distinction of subsistence and substance to show that God is through his substance one but in subsistence of the persons three. Bernhard however considers this formal differentiation a material one and thus gets Gilberts intention wrong.