Hear what Truth says: No one knows the Son except the Father, nor does anyone know the Father except the Son (Matt. 11:27). And if no one attains this knowledge in this present age, whence does the Arian derive a measure for dividing the very substance? If the Father alone knows the Son, and the Son alone the Father, whence does the blind guide understand which is greater or lesser? Or perhaps he answers: even if the Godhead alone reserves to itself the secret of this knowledge, whence have you come to know the Son as equal to the Father? But I say: the Son proclaims fatherhood, the creature bears witness to omnipotence; but as for the innermost depth of the substance, if no one knows the Son except the Father, nor does anyone know the Father except the Son —
— where the knowledge is equal, exchanged by mutual counsel, there is without doubt one nature in the perfection of the substance. For he is neither greater nor lesser who is neither less nor more fully known. What now shall we say of the Holy Spirit, whom we are commanded to believe in the Trinity by the following clause of the creed? For if this knowledge unites the Father and the Son to each other, the Holy Spirit will not be a stranger to it, of whom it is written that the Spirit searches all things, even the depths of God (1 Cor. 2:10). Therefore not only do the Father and the Son know themselves, but also their Spirit; from whom nothing at all is hidden, because nothing is truly known apart from Him. Wherefore knowledge is even as the cooperation of the Trinity.
For after this mystery of the divine generation, if we consider the human things in Christ — that we celebrate that the Virgin gave birth — the Father sent, the Son came, the Holy Spirit poured forth. Wherefore neither is the brightness of equality hidden in the lowliness of being made less.