[Tertullian] also spoke of the unity in the trinity, denying any form of tritheistic tendencies. Instead, he speaks of “economy”, an important word in ancient Christian theology. To speak of divine economy is to speak of God “building up” his manifestations in periods of history. In a living and dynamic way the trinity is built up in historical manifestations. But in this trinity there is but one divine essence.
If we translate “essence” by “power of being”, we have what this word meant. There is one divine power of being, and each of the three economic manifestations of the power of being participates in the full power of being.
Paul Tillich, A History of Christian Thought (Simon&Schuster: 1967) p. 46