Ante-Nicene Fathers, Vol III:
Tertullian: Part II: The Designation of the One God in the Prophetic Scriptures. Intended as a Protest Against Heathen Idolatry, It Does Not Preclude the Correlative Idea of the Son of God. The Son is in the Father.

Chapter XVIII.—The Designation of the One God in the Prophetic Scriptures. Intended as a Protest Against Heathen Idolatry, It Does Not Preclude the Correlative Idea of the Son of God. The Son is in the Father.

But what hinders them from readily perceiving this community of the Father’s titles in the Son, is the statement of Scripture, whenever it determines God to be but One; as if the selfsame Scripture had not also set forth Two both as God and Lord, as we have shown above. 7985 Their argument is: Since we find Two and One, therefore Both are One and the Same, both Father and Son.  Now the Scripture is not in danger of requiring the aid of any one’s argument, lest it should seem to be self-contradictory. It has a method of its own, both when it sets forth one only God, and also when it shows that there are Two, Father and Son; and is consistent with itself. It is clear that the Son is mentioned by it.  For, without any detriment to the Son, it is quite possible for it to have rightly determined that God is only One, to whom the Son belongs; since He who has a Son ceases not on that account to exist,—Himself being One only, that is, on His own account, whenever He is named without the Son. And He is named without the Son whensoever He is defined as the principle (of Deity) in the character of “its first Person,” which had to be mentioned before the name of the Son; because it is the Father who is acknowledged in the first place, and after the Father the Son is named. Therefore “there is one God,” the Father, “and without Him there is none else.” 7986 And when He Himself makes this declaration, He denies not the Son, but says that there is no other God; and the Son is not different from the Father. Indeed, if you only look carefully at the contexts which follow such statements as this, you will find that they nearly always have distinct reference to the makers of idols and the worshippers thereof, with a view to the multitude of false gods being expelled by the unity of the Godhead, which nevertheless has a Son; and inasmuch as this Son is undivided and inseparable from the Father, so is He to be reckoned as being in the Father, even when He is not named. The fact is, if He had named Him expressly, He would have separated Him, saying in so many words: “Beside me there is none else, except my Son.” In short He would have made His Son actually another, after excepting Him from others.  Suppose the sun to say, “I am the Sun, and there is none other besides me, except my ray,” would you not have remarked how useless was such a statement, as if the ray were not itself reckoned in the sun? He says, then, that there is no God besides Himself in respect of the idolatry both of the Gentiles as well as of Israel; nay, even on account of our heretics also, who fabricate idols with their words, just as the heathen do with their hands; that is to say, they make another God and another Christ. When, therefore, He attested His own unity, the Father took care of the Son’s interests, that Christ should not be supp. 614 posed to have come from another God, but from Him who had already said, “I am God and there is none other beside me,” 7987 who shows us that He is the only God, but in company with His Son, with whom “He stretcheth out the heavens alone.” 7988

Footnotes

See above ch. xiii. p. 607.

Isa. xlv. 5.

Isa. 45:5, 18, Isa. 44:6.

Isa. xliv. 24.