7 For it was fitting that the truth should receive testimony from all, and should become [a means of] judgment for the salvation indeed of those who believe, but for the condemnation of those who believe not; that all should be fairly judged, and that the faith in the Father and Son should be approved by all, that is, that it should be established by all [as the one means of salvation], receiving testimony from all, both from those belonging to it, since they are its friends, and by those having no connection with it, though they are its enemies.
For that evidence is true, and cannot be gainsaid, which elicits even from its adversaries striking testimonies in its behalf; they being convinced with respect to the matter in hand by their own plain contemplation of it, and bearing testimony to it, as well as declaring it. But after a while they break forth into enmity, and become accusers [of what they had approved], and are desirous that their own testimony should not be [regarded as] true. He, therefore, who was known, was not a different being from Him who declared “No man knows the Father,” but one and the same, the Father making all things subject to Him; while He received testimony from all that He was very man, and that He was very God, from the Father, from the Spirit, from angels, from the creation itself, from men, from apostate spirits and demons, from the enemy, and last of all, from death itself.
But the Son, administering all things for the Father, works from the beginning even to the end, and without Him no man can attain the knowledge of God. For the Son is the knowledge of the Father; but the knowledge of the Son is in the Father, and has been revealed through the Son; and this was the reason why the Lord declared: “No man knows the Son, but the Father; nor the Father, save the Son, and those to whomsoever the Son shall reveal [Him].” For “shall reveal” was said not with reference to the future alone, as if then [only] the Word had begun to manifest the Father when He was born of Mary, but it applies indifferently throughout all time.
For the Son, being present with His own handiwork from the beginning, reveals the Father to all; to whom He wills, and when He wills, and as the Father wills. Wherefore, then, in all things, and through all things, there is one God, the Father, and one Word, and one Son, and one Spirit, and one salvation to all who believe in Him.
Source: Adversus haereses (New Advent)