There are three monotheistic religions in the world: Judaism, Christianity, and Mahommedanism. The second is a development of the first; the third is an outgrowth of both.
The doctrine of the Unity of God is held in contradistinction to Polytheism, which is belief in a multiplicity of gods; Tri-theism, which teaches that there are three Gods—that is, that the Father, the Son, and the Holy Ghost are, specifically, three distinct Gods; and to Dualism, which teaches that there are two independent divine beings or eternal principles, the one good, and the other evil, as set forth especially in Gnostic systems, such as Parseeism.
a) The Scriptures Assert the Unity of God.
Deut. 6:4—"Hear, O Israel; the Lord our God is one Lord"; or, "The Lord our God, the Lord is one." Isa. 44:6-8—"First…. last…. beside me there is no God." Isa. 45:5—"There is none else, there is no God beside me." 1 Tim. 2:5 "There is one God." 1 Cor. 8:4—"There is none other God but one."
That God is one, that there is no other, that He has no equal is the forceful testimony of above fifty passages in the Scriptures. The fundamental duty of life, namely, the devotion of the entire being to the Lord, is based upon the Unity of God: "The Lord….is one …. therefore thou shalt love the Lord thy God with all thy heart," etc.
No other truth of the Scripture, particularly of the Old Testament, receives more prominence than that of the Unity of God. This truth is clearly pronounced also in the material universe; it is the introduction and conclusion of all scientific researches. Any other representation contradicts both creation and revelation. Its denial is a proper object for the ridicule of every thinking man, and of the disbelief of every orthodox Christian. Let this, then, be our first and necessary conclusion—that Deity, whether creating, inspiring, or otherwise manifesting itself, is one God; one, and no more.—Cerdo.
A multiplication of Gods is a contradiction; there can be but one
God. There can be but one absolutely perfect, supreme, and almighty
Being. Such a Being cannot be multiplied, nor pluralized. There
can be but one ultimate, but one all-inclusive, but one God.
Monotheism, then, not Tri-theism, is the doctrine set forth in the Scriptures. "If the thought that wishes to be orthodox had less tendency to become tri-theistic, the thought that claims to be free would be less Unitarian."—Moberly.
b) The Nature of the Divine Unity.
The doctrine of the Unity of God does not exclude the idea of a plurality of persons in the Godhead. Not that there are three persons in each person of the Godhead, if we use in both cases the term person in one and the same sense. We believe, therefore, that there are three persons in the Godhead, but one God. Anti-trinitarians represent the evangelical church as believing in three Gods, but this is not true; it believes in one God, but three persons in the Godhead.