1 John 5:20

When ye read, "That we may be in His true Son Jesus Christ," think of the "true Son" of God. But this Son ye in no wise think to be the true Son of God, if ye deny Him to be begotten of the substance of the Father. For was He already Son of Man and by gift of God became Son of God, begotten indeed of God, but by grace, not by nature? Or, though not Son of Man, yet was He some sort of creature which, by God's changing it, was converted into Son of God? If you mean nothing of this sort, then was He either begotten of nothing, or of some substance. But thou hast relieved us from all fear of having to suppose that you affirm the Son of God to be of nothing, for thou hast declared that this is not your meaning. Therefore, He is of some substance. If not of the substance of the Father, then of what? Tell me. But ye cannot find any other. Consequently, the Father and the Son are of one and the same substance. This is the Homousion.

In the Scriptures both you and We read, "That we may be in His true Son Jesus Christ; He is the true God and Eternal Life." Let both parties yield to such weighty evidence. Tell us then, whether this "true Son" of God, discriminated as He is by the property of this name from those who are sons by grace, be of no substance or of some substance. Thou sayest, "I do not say that He is of no substance, lest I should say that He is of nothing." He is therefore of some substance: I ask, of what? If not of the substance of the Father, seek another. If thou findest not another, as indeed thou canst find none at all, then acknowledge it to be the Father's, and confess the Son Homousios, "of one substance with the Father."

Flesh is begotten of flesh, the Son of flesh is begotten of the substance of the flesh. Set aside corruption, reject from the eye of the mind all carnal passions, and behold "the invisible things of God understood by the means of the things that are made." Believe that the Creator who hath given flesh power to beget flesh, who hath given parents power of the substance of the flesh to generate "true sons" of flesh, much more had power to beget a "true Son" of His own substance, and to have one substance with the true Son, the spiritual incorruption remaining and carnal corruption being altogether alien therefrom.

If He is begotten, He is Son: if He is Son, He is the "true Son," because Only-Begotten. For we also are called sons: He Son by nature, we sons by grace. To say that because He is begotten, He is of another nature, is to deny that He is the "true Son." Now we have the Scripture: "That we may be in His true Son Jesus Christ; He is the true God and Eternal Life." Why "true God"? because "true Son" of God. For if He has given to animals this property, that what they beget shall be none other than what they themselves are: man begets man, dog begets dog, and should God not beget God? If then He is of the same substance, why callest thou Him less?

Is it because when a human father begets a son, though human beget human, yet greater begets less? If so, then let us wait for Christ to grow as human beings grow whom human beings beget! But if Christ, ever since He was begotten (and this was not in time but from eternity), is what He is, and yet is less than the Father, at that rate the human condition is the better of the two: for a human being at any rate can grow, and has the property of sooner or later attaining to the age, to the strength of the father; but He never: then how is He a "true Son"?