St. Hilary of Poitiers (ca. 315-68) in his De Trinitate discusses the relationship between God the Father and His Son. Here are a few highlights of his teaching:
St. Hilary writes: “It is the Father to Whom all existence owes its origin. In Christ and through Christ He is the source of all. In contrast to all else He is self-existent. He does not draw His being from without, but possesses it from Himself and in Himself. He is infinite, for nothing contains Him and He contains all things; He is eternally unconditioned by space, for He is illimitable. . . . He does not, as men do, receive the power of paternity from an external source. He is unbegotten, everlasting, inherently eternal. To the Son only is He known, for no one knoweth the Father save the Son and him to whom the Son willeth to reveal Him (Matt 11:27).” (II, 6)
Christ said, “The Father is in Me and I in the Father” (John 10:38). St. Hilary elaborates: “As the Father is Spirit, so is the Son Spirit; as the Father is God, so is the Son God; as the Father is Light, so is the Son Light. Thus those properties which are in the Father are the source of those wherewith the Son is endowed. . . . The Father is in the Son, for the Son is from Him; the Son is in the Father, because the Father is His sole Origin; the Only-begotten is in the Unbegotten, because He is the Only-begotten from the Unbegotten. Thus mutually Each is in the Other.” (III, 4)
The centurion, the guardian of the cross, cried out, “Truly this was the Son of God” (Matt 27:54). St. Hilary comments: “Multitudes of us are sons of God; He is Son in another sense. For He is God’s true and own Son, by origin and not by adoption.” (III, 11)
A person is called a “father” precisely because he has a son: “The name of Father involves that of Son, since without having a son none can be a father. Evidence of the existence of a son is proof that there has been a father, for a son cannot exist except from a father. When we confess that God is One we deny that He is single; for the Son is the complement of the Father, and to the Father the Son’s existence is due.” (VII, 31)
Christ said, “I and the Father are One” (John 10:30), and “He that hath seen Me hath seen the Father also” (John 14:9). St. Hilary remarks: “A recognition of God the Son produces a recognition of God the Father. For the Son is in such a sense the Image, as to be One in kind with the Father, and yet to indicate that the Father is His Origin. . . . The Son that is born of the Father has a nature in no wise different from His; and, because His nature is not different, He possesses the power of that nature which is the same as His own.” (VII, 37)
Quotations from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Vol. IX, ed. Philip Schaff and Henry Wace (Edinburgh: T&T Clark, 1886).