De Trinitate, lib. i. 6, § 9.
And if the Son be not of the same substance as the Father, then is He a made substance: if a made substance, then not “all things were made by Him:” but, “all things were made by Him;”1 John i. 2. therefore, He is of one and the same substance with the Father. And therefore, not only God, but True (or, Very) God. Which the same John doth most openly affirm in his epistle: “Scimus quod Filius Dei venerit et dederit nobis intellectum ut cognoscamus verum Deum, et simus in vero Filio ejus Jesu Christo. Hic est verus Deus et vita æterna.” “We know that the Son of God is come; and hath given us an understanding that we may (learn to) know the True God,2 So τὸν ἀληθινὸν Θεόν. St. Basil, St. Cyril. Al. Vers. Arab. Aeth. Cod. Al. (ΑΛΗΘΕΙΝΟΝΘΝ, which abbreviated manner of writing may explain the omission) and several other mss. Beda, verum Deum. Facundus: quod est verum (τὸ ἀληθινόν). and may be in His true Son Jesus Christ. This is the True God and Eternal Life.”
10. Hence also by consequence we understand, that what the apostle Paul saith, “Who only hath immortality,”3 1 Tim. i. 16. he saith not merely of the Father, but of the One and Only God, which the Trinity itself is. For neither is the “Eternal Life” itself mortal in respect of any mutability: and consequently, since the Son of God “is Eternal Life,” He also is to be understood together with the Father, where it is said, “Who only hath immortality.”