According to Holy Scripture, God is one in essence, but in this one essence there are three distinct Persons, Father, Son, and Holy Ghost. This is the Christian doctrine of God. (Luther: Scriptura Sancta docet esse Deum simplicissime unum et tres, ut vocant, personas verissime distinctas." (St. L., X, 176 f.)) This doctrine of Scripture the Christian Church expresses by the term "Trinity."
That God is one in essence, though three in person, Holy Scripture teaches distinctly; its doctrine of God in both the Old and the New Testament is exclusively monotheistic. According to Scripture, God is one, and besides this one God there is no God. (Deut. 6, 4: ''Hear, O Israel: the Lord, our God, is one Lord"; 1 Cor. 8, 4: "There is none other God but one.") All idols of the heathen Holy Scripture designates as "non-gods," Jer. 2, 11; "vanities," אֱלִילִם Lev. 19, 4, or as things which are wholly without real existence. Cp. the descriptions of idols in Is. 44, 6-20; J er. 2, 26-28; Ps. 115, 1-9; 135, 15-17. In the New Testament St. Paul writes with equal emphasis: "An idol is nothing in the world," 1 Cor. 8, 4, and from this he draws the basic Christian doctrine : "But to us there is but one God," 1 Cor. 8, 6.
With the paramount truth of God's existence Holy Scripture immediately connects the demand of divine adoration. The one true God, who has revealed Himself in His Word, must be adored and served by all men. (Ex. 20, 3: "Thou shalt have no other gods before me"; Mark 12, 29. 30: "And Jesus answered him, The first of all the commandments is, Hear, 0 Israel: The Lord, our God, is one Lord. And thou shalt love the Lord, thy God, with aU thy heart, and with all thy soul, and with all thy mind, and with all thy strength.") As polytheism eliminates the very concept of God, so also it destroys true worship. Hence, if the heathen would worship God, they must turn from their idols to the true God. (Acts 14, 15: "Ye should turn from these vanities, ἀπὸ τούτων τῶν ματαίων, unto the living God.")
However, while Holy Scripture most earnestly inculcates the doctrine of God's unity, it teaches at the same time that the one God is the Holy Trinity. When Christ sent forth His disciples to teach all nations, He expressly commanded them to baptize "in the name of the Father and of the Son and of the Holy Ghost," Matt. 28, 19. However, "Father, Son, and Holy Ghost" are designations of three persons; hence the Christian Church teaches, on the basis of Scripture: "God is one, and yet in the one divine essence are three distinct persons." ("Father, Son, and Holy Ghost, three distinct persons in one divine essence and nature, are one God, who has created heaven and earth," Smalcald Art., First Part.) As Holy Scripture connects with the doctrine of the unity of God the demand that this one God must be worshiped, so also it demands that the one true God should be adored as the Holy Trinity. In other words, not one Person of the Godhead should be worshiped, but all three. (1 John 2, 23: "Whosoever denieth the Son, the same hath not the Father"; 5, 12: "He that hath not the Son hath not life"; John 5, 23: "All men should honor the Son even as they honor the Father. He that honoreth not the Son honoreth not the Father which hath sent Him.") On the basis of this clear doctrine of Scripture the Apology of the Augsburg Confession affirms (Art. I): "We declare that we believe and teach that there is one divine essence, undivided, etc., and yet, that there are three distinct persons, of the same divine essence and coeternal, Father, Son, and Holy Ghost. This article we have always taught and defended, and we believe that it has, in Holy Scripture, sure and firm testimonies that cannot be overthrown. And we constantly affirm that those thinking otherwise are outside the Church of Christ and are idolaters and insult God."
In order that we may hold to the pure Scriptural doctrine concerning the Holy Trinity, we must maintain, on the basis of Scripture, that each Person in the Godhead is the entire God (tot'U8 Deus), or that each Person has the whole divine essence without division or multiplication (sine divisione et multiplicatione). "Of these Persons each one is the whole God, besides whom there is no other God." Luther. By the expression sine divisione we mean to say that the divine essence with its attributes is not divided among the three Persons, so that the Father has one-third, the Son onethird, and the Holy Ghost one-third, but that each Person has the whole divine essence entire and undivided. This is not a "dogmatical construction," but Scriptural doctrine. Col. 2, 3 : "In whom are hid all the treasures of wisdom and knowledge"; 2, 9 : "In Him (Christ) dwelleth all the fulness of the Godhead bodily." By the expression sine multiplicatione we declare that there are not three distinct sets, or series, of divine attributes, so that the Father has one set, the Son another, and the Holy Ghost a third, but that the one and same essence with all its divine attributes belongs to the Father, the Son, and the Holy Ghost, according to number (nurnero, secundum numerum), not merely according to kind (specie). Of men it must be said that there are as many essences as there are persons (quot personae, tot essentiae), but of God, Holy Scripture testifies that the three Persons of the Godhead have one and the same essence with all its attributes numerically. Tres personae, una essentia aivina, individua, unus Deus. This sublime truth Holy Scripture teaches in the following passages: John 10, 30: "I and My Father are one (εv)"; John 5, 1 'l: "My Father worketh hitherto, and I work"; 5, 19 : "The Son can do nothing of Himself but what He seeth the Father do; for what things soever He doeth, these also doeth the Son likewise"; 10, 37: "If I do not the works of My Father, believe Me not." This truth the Creed of Athanasius professes as follows: "We worship one God in Trinity, and Trinity in Unity; neither confounding the Persons nor dividing the Substance. For there is one Person of the Father; another of the Son, and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost is all one: the glory equal, the majesty coeternal. Such as the Father is, such is the Son, and such is the Holy Ghost. . . . So likewise the Father is almighty, the Son almighty, and the Holy Ghost almighty. And yet they are not three Almighties, but one Almighty. So the Father is God, the Son is God, and the Holy Ghost is God. And yet they are not three Gods, but one God .... And in this Trinity none is before or after other; none is greater or less than another; but the whole three Persons are coeternal together and coequal: so that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshiped." Triglot, p.33.1
The Scriptural doctrine of the Holy Trinity is absolutely incomprehensible to the human mind; for on the basis of Scripture we profess one undivided and indivisible God, so that each Person is the entire God (totus Deus); and yet three really distinct Per sons, so that, when the Son became incarnate, He alone became man and not the Father or the Holy Ghost, and when the Son suffered and died, He alone suffered and died and not the Father or the Holy Ghost. This truth is beyond reason, for according to reason the Unity annuls the Trinity and the Trinity the Unity. In other words, human reason must assume either one God or three Gods. It cannot reconcile Unity with Trinity nor Trinity with Unity. Consequently all errorists on this point have denied either the Unity or the Trinity.