Both the scriptures and the early church fathers call the Father alone the “one God” (see: I believe in one God, the Father Almighty). The reason for this is because the person of the Father alone is the supreme uncaused Cause of all (see: Why There is Only One God: One Supreme Cause), and the one Supreme Authority over all (see: Why There is Only One God: Headship). Yet scripture is also clear in teaching that the Son and Holy Spirit have the same divine nature as the Father (see: Why There is Only One God: One Divine Nature).
Upon noting the distinctions between the persons, and the incommunicable attributes of the Father which the Son and Spirit do not share (being the supreme uncaused Cause of all, being the Supreme Authority over all, and being Father), some have wondered: do the incommunicable attributes of the Father show Him to be of a different nature than the persons of the Son and Holy Spirit?
This question is an ancient one, and one that needed to be addressed in the trinitarian debates of the ancient church. Arians taught that the Son and Spirit are indeed of a different nature than the Father; they often cited the Father’s attribute of being unoriginate as a reason why the Son and Spirit could not be of the same nature with Him. God is unoriginate in His nature, they would argue. The Son and Holy Spirit, on the other hand, are not unoriginate, as They both have the Father as Their origin. Thus, the Arian would conclude, since the Son and Spirit lack this important attribute of divinity, They cannot possibly be of the same divine nature as the Father.
The short answer to this flawed line of reasoning is that we need to distinguish between relative attributes, and essential attributes. Relative (or relational) attributes tell us about the relationship between the persons, but not what They are in Their nature. Essential attributes, on the other hand, describe what the persons are in Their nature.
One way of illustrating this distinction would be that the difference between essential attributes and relative attributes can be viewed as the same difference as between the answers to the questions “what?” and “why?”
When we ask “what” the persons are, the answer is the same for all three persons: God, that is, the divine nature. What is the Father? God. What is the Son? God. What is the Spirit? God. But when we ask “why” They are each God, the answers are unique for each person. Why is the Father God? He simply is, without cause or origin of any kind. Why is the Son God? Because He was begotten of the Father before the ages. Why is the Spirit God? Because He eternally proceeds from the Father.
These relative, or relational attributes, of being uncaused, begotten, and proceeding distinguish the persons from one another, but they ultimately aren’t telling us what the persons are. Essential attributes, on the other hand, do tell us what the persons are: love, justice, mercy, power, goodness, holiness, perfection, etc.
This distinction is a necessary one, not only as being logically necessary, but also in order to be able to account for the scripture’s teachings on the Son and Spirit having the same divine nature as the Father. Sonship necessarily implies sameness of nature; thus when the Jews heard Christ claim to be the Son of God, they accused Him of claiming to be God (John 10). This is because they understood the implication of the teaching that Jesus was the Son of God- every Son has the same nature as His Father. Every time scripture teaches that the Son is the Son of God -not by adoption as we, but only-begotten -it is proclaiming the co-divinity of the Son with the Father.
On the other hand, if we say that the Son does not have the same nature as the Father, we will ultimately be denying the true sonship of the Son by extension; and by further extension, would be denying the fatherhood of the Father. Those then who seek to defend the Father’s unique prerogatives as Father by denying the exact identicality of His divine nature with that of the Son and Holy Spirit err greatly, and actually work against their own purpose by effectively denying the Father’s fatherhood. We must rather affirm, as scripture teaches, that there is one God -the Father. We cannot affirm that He is one God in such a way as to deny that He is Father, nor teach that He is Father while denying Him the prerogative of being the one God.