Paul Tillich, A History Of Christian Thought
Neo-Platonism: Plotinus. Clement of Alexandria. Origen.
Neo-Platonism is not only important because it was the philosophy which deeply influenced the first great theological system, that of Origen, but it was also the philosophy which influenced (through Dionysius the Areopagite, of whom we shall hear more later) all forms of Christian mysticism and most forms of classical Christian theology, especially with respect to the doctrine of God, world, and soul.
Therefore it is impossible to understand the development of Christian theology without knowing something about this last great attempt of paganism to express itself in terms of a philosophical theology, or theological philosophy, which was both science and life for the ancient mind.
The man who is mostly responsible for the system of Neo-Platonism is Plotinus, who according to his dependence on Plato, is called “neo-Platonist”; but it is not he alone, it is a whole school of greatest influence. There is not only a scientific and religious side but also a political side to it: the emperor Julian the Apostate tried to introduce, against Christianity, the Neo- Platonic system, which shows that he considered it not only as a science but as the all-embracing system of religious elevation of the soul. All these things make it necessary to dwell on this system more than perhaps you think it necessary, for a philosophical non-Christian system.
God, for Plotinus, is the transcendent One, the One which transcends every number; also the number “one” insofar as it is a number which includes 2, 3, 4, 5, etc. It is that which is beyond number, and for this he uses the word “one.” So when you hear, in all mystical language through all the centuries, the word “one” in the mystical expressions, don’t take it as one beside others, but as that which transcends numbers.