Protestants fiercely defended the traditional Christian doctrine of God during the time of the Reformation. The writings of the Reformers are filled with defenses of the Trinity and arguments against contemporary heretical groups such as the Socinians, who denied the deity of Christ. Part and parcel of their defense of the Trinity, however, is the recognition that merely defending the deity of Christ is not enough to give us the biblical doctrine of God. More must be said about how Christ, who is God incarnate, is related to God the Father, who has not taken on flesh.
Thus, the Reformers, like the early church fathers, turned to John 1:1–18 not only to demonstrate the deity of Christ but to prove His distinction from the Father. As we see in verses 1–2 of today’s passage, the Word—the Son of God—in the beginning was God and was with God. John introduces a distinction between God the Father and God the Son. Both are equally God and yet the Father is not the Son. Each possesses the full complement of divine attributes, but each in some way also has a particular identity. As John Calvin comments, “It would have been absurd in the Evangelist to say that the [Word] was always with God, if he had not some kind of subsistence peculiar to himself in God.”
Over time, the church came to use the Greek word hypostasis, which we usually translate as “person,” to refer to the distinctions within the one God. The Reformers adopted this terminology because it is a helpful way of describing the multiplicity Scripture tells us belongs to the Godhead. The hypostasis of the Father is not the hypostasis of the Son, but both hypostases are homoousios (of the same essence).
This language is helpful, but we must note that it does not eliminate the mystery inherent to God. Our Creator is ultimately, but not totally, incomprehensible. We can know true things about Him, but we cannot know everything about Him. We cannot know Him as He knows Himself. It is difficult to define what we mean by person when we talk about the three persons of the Godhead, for in theological language, person is not identical to our modern concept of personhood. We are on safe ground to say little more than this: personhood in the Godhead means that while there is no difference between Father, Son, and Holy Spirit in terms of deity, there are still distinctions between the persons that enable Them to enjoy personal relations with one another, to love and be loved by the other persons.