“Simplicity” in respect to God may sound odd to those not familiar with the historic use of the term and the doctrine it denotes. God is beyond our comprehension, infinite, and transcendent; when we speak of God’s ‘simplicity’, we are not saying that God is easily comprehended by our minds. Rather, ‘simple’ here is being used as the opposite of compound; the idea is that God does not have parts that He is composed of.
This of course fits with the fact that God is incorporeal, and infinite, etc. But even beyond imagining God as somehow being composed of physical or material parts, some are inclined to think of the attributes of God as parts of Him. Some will speak of God as if He is part holy, and part loving, as though God were a blend of many individual parts. This however, is not how the scriptures reveal God.
The idea of simplicity is that rather than being composed of parts, God is what He is. For example, God is just; therefore, He is, in His very nature, the definition of justice. God is good; He is the very definition of goodness. God is loving; therefore, God is love. We can follow this pattern with a whole list of God’s attributes.
Scripture speaks of the simplicity of God, for example, when it says “God is love” (1 John 4:8). And the idea that God is what He is is expressly stated in Exodus 3:14 “And God saith unto Moses, `I Am That Which I Am;’ He saith also, `Thus dost thou say to the sons of Israel, I Am hath sent me unto you.'” (YLT).
This conceptually makes a lot of sense when we begin considering the attributes of God. Take love, for example. It is intangible, and eternal. Love is not something created by God that did not exist at one point and came into existence later; in fact, if any attribute of God came into existence as a creation of God, God would have changed, a thing which is impossible: ““For I am the Lord, I do not change; Therefore you are not consumed, O sons of Jacob.” (Mal 3:6 NKJV). So we know that God has always had all His attributes, without change. That means that they are eternal.
But they do not exist eternally as something separate from God that is coeval with Him. Before creation, there was only God, His Son, and His Spirit; these attributes exist eternally as what God is. Love, justice, truth, and perfection exist eternally as what God is in His very nature. The essential attributes of God exist eternally as what God is.
When we examine this relative to the classical trinitarianism scripture teaches, we understand that Father as the one God, Who is what He is of Himself. In what He is, He is the very definition of goodness, love, justice, and perfection. His simple divine nature is properly His own; He has it from no other source. And this simple divine nature is communicated to His only-begotten Son in His eternal generation, so that the Son has the same simple divine nature. The Holy Spirit likewise, eternally from the Father by procession, has the Father’s simple divine nature as His own.
Each person then, is the very definition of love, goodness, justice, etc in their very nature, for they all possess the same simple divine nature, the Father possessing it unoriginately, and the Son and Spirit participating in the Father’s divine nature. Thus there is one God, Who alone is good; yet His Son and Spirit participate in this same goodness. So there are three persons who are good, each in their nature the very definition of goodness; yet there is only one goodness, the one essence they share.
Semi-modalism, however, stumbles and falls to pieces over the doctrine of simplicity. Semi-modalism, unlike classical trinitarianism, posits one divine person who is the three divine persons of the Father, Son, and Spirit. However, since the divine nature is simple, how can this one person be three persons? The super-person, “God the Trinity”, cannot be simple, since he is compound of the three real persons of the Trinity, in the vain imagination of the semi-modalists. For in simplicity, all of God’s attributes are equated with one another; God’s nature is equally the very definition of both love and justice, for example. In God, love and justice are not two different things, but both are what God is in his nature.
However, in semi-modalism, this does not work. For semi-modalists posit a real relational distinction between the persons of the Father, Son, and Spirit, so that the persons are not equated with one another. This is why they are semi-modalists, not classical modalists like Sabellius. However, if these persons are not equated, yet are all together a single person, then this single person is unlike and unequal to himself, and not at all simple. “God the Trinity” then, since he is both the Father and the Son, is imagined to be both unbegotten as Father and begotten as Son, and proceeding as Spirit. What absurdity, what blasphemous heresy, teaches a God who is a Father-Son, begetter and begotten of his own self? When examining this it is difficult to see a meaningful difference between the false teaching of the semi-modalists and that articulated by Sabellius himself. For all they have done is resurrected his heresy in a slightly modified form.
Yet if they will say that their super-person, their “triune God”, is simple, then He cannot have parts, and the things he is must be identical to one another; this then would be a denial of any real personal distinctions between the persons of the Trinity, and bring them to be classical modalists, since the Father, Son, and Spirit then must be regarded as identical with each other. Yet if they deny that their imagined person of the “triune God” is simple, then he cannot be regarded as God at all, since the divine nature is simple. How blasphemous then they would be to give worship to a person who is not true God, and to make him out to be the real persons of the Trinity! Or else perhaps they will posit the absurdity that he is God in nature, yet not simple. If this is the case, then he possesses not the same divine nature as the real persons of the Trinity; and having introduced a second divine nature, they convict themselves of being polytheists, believing in multiple gods.
Such then, is the absurdity of the semi-modalists, whose pseudo-trinitarianism is incompatible with scripture’s teaching of the simplicity of the divine nature. Let us, however, hold both simplicity and the doctrine of the Trinity as being equally true, and entirely compatible, as we have seen above, how unlike semi-modalism, classical trinitarianism is entirely compatible with the doctrine of divine simplicity.