No one is to be called a Theist, who does not believe in a Personal God, whatever difficulty there may be in defining the word “Personal.” Now it is the belief of Catholics about the Supreme Being, that this essential characteristic of His Nature is reiterated in three distinct ways or modes; so that the Almighty God, instead of being One Person only, which is the teaching of Natural Religion, has Three Personalities, and is at once, according as we view him in the one or the other of them, the Father, the Son, and the Spirit–a Divine Three, who bear towards Each Other the several relations which those names indicate, and are {125} in that respect distinct from Each Other, and in that alone.
John Henry Newman
An Essay in aid of a Grammar of Assent, Chapter 5