You who are without sense and in all things reckless, why do you not the rather cease your impertinent inquiries about the holy Trinity, and only believe that it exists ? You have the Apostle as your teacher for this, when he says: "It is necessary first to believe on God that he is, and that he is a rewarder of them that seek after him." (Cf Heb 11) He did not say, "how he is," but only, "that he is".' But if they are not overwhelmed by this, let them say how the Father is, that so they may learn how his Word is. But it is absurd, they will say, to ask such questions about the Father. Let them hear, then, that it is also absurd to ask them concerning his Word.
Since, therefore, such an attempt is futile madness, nay, more than madness !, let no one ask such questions any more, or else let him learn only that which is in the Scriptures. For the illustrations they contain which bear upon this subject are sufficient and suitable. The Father is called fountain and light: "They have forsaken me the fountain of living water";(Jer 2:13) and again in Baruch, "Why, O Israel, art thou in the land of thine enemies ? Thou hast forsaken the fountain of wisdom"; (Bar 3:12) and, according to John: "Our God is light." (1Jo 1:5) But the Son, in contrast with the fountain, is called river: "The river of God is full of water." (Ps 65:9) In contrast with the light, he is called radiance, as Paul says: "Who, being the radiance of his glory and the image of his essence." (Heb 1:3) We may see in the Son the Spirit also by whom we are enlightened. "That he may give you the Spirit of wisdom and revelation in the knowledge of him, having the eyes of your heart enlightened". (Eph 1:17-18) But when we are enlightened by the Spirit, it is Christ who in him enlightens us. For it says: "There was the true light which lighteth every man coming into the world." (Joh 1:9) Again, as the Father is fountain and the Son is called river, we are said to drink of the Spirit. For it is written: "We are all made to drink of one Spirit." (1Co 12:13) But when we are made to drink of the Spirit, we drink of Christ. For "they drank of a spiritual rock that followed them, and the rock was Christ" (1Co 10:4)
Again, as Christ is true Son, so we, when we receive the Spirit, are made sons. "For you have not received the spirit of bondage again to fear; but you have received the Spirit of adoption." (Ro 8:15) But if by the Spirit we are made sons, it is clear that it is in Christ we are called children of God. For: "So many as received him, to them gave he the power to become children of God." (Joh 1:12)
Then, as the Father, in Paul's words, is the "only wise", (Ro 16:27) the Son is his Wisdom: "Christ the Power of God and the Wisdom of God." (1Co 1:24) But as the Son is Wisdom, so we, receiving the "Spirit of Wisdom", (Eph 1:17) have the Son and are made wise in him. For thus it is written in the one hundred and forty-fifth psalm: "The Lord looseth the prisoners, the Lord maketh wise the blind." (Ps 145 7-8 Sept.) When the Holy Spirit is given to us ("Receive the Holy Spirit," said the Saviour), God is in us; for so John wrote: "If we love one another, God abideth in us ; hereby know we that we abide in him and he in us, because he hath given us of his Spirit." (1Jo 4:12-13) But when God is in us, the Son also is in us. For the Son himself said: "The Father and I will come and make our abode with him." (Joh 14:23)
Furthermore, as the Son is life — for he says "I am the life" (Joh 11:25) — we are said to be quickened by the Spirit. For it says: "He that raised up Christ Jesus from the dead shall quicken also your mortal bodies, through his Spirit that dwelleth in you. (Ro 8:11) But when we are quickened by the Spirit, Christ himself is said to live in us; for it says: "I have been crucified with Christ. I live, and yet no longer I, but Christ liveth in me." (Ga 2:20)
Again, the Son declared that the Father worked the works that he did — for he says: "The Father abiding in me doeth his works. Believe me, that I am in the Father and the Father in me; or else believe me for his works' sake." (Joh 14:11) So Paul declared that the works he worked by the power of the Spirit were the works of Christ: "For I will not dare to speak of any things save those which Christ wrought through me, for the obedience of the Gentiles, by word and deed, in the power of signs and wonders, in the power of the Holy Spirit." (Ro 15:18-19)
But if there is such co-ordination and unity within the holy Trinity, who can separate either the Son from the Father, or the Spirit from the Son or from the Father himself? Who would be so audacious as to say that the Trinity is unlike itself and diverse in nature, or that the Son is in essence foreign from the Father, or the Spirit alien from the Son? But how are these things ? If one should make inquiry and ask again : How, when the Spirit is in us, the Son is said to be in us ? How, when the Son is in us, the Father is said to be in us ? Or how, when it is truly a Trinity, the Trinity is described as one? Or why, when the One is in us, the Trinity is said to be in us ? — let him first divide the radiance from the light, or wisdom from the wise, or let him tell how these things are. But if this is not to be done, much more is it the audacity of madmen to make such inquiries concerning God. For tradition, as we have said, does not declare the Godhead to us by demonstration in words, but by faith and by a pious and reverent use of reason. For if Paul proclaimed the saving Gospel of the Cross, "not in words of wisdom, but in demonstration of the Spirit and of power" (1Co 2:4) ; and if in Paradise he heard "unspeakable words which it is not lawful for a man to utter" (2Co 12:4) : who can declare the holy Trinity itself? Nevertheless, we can meet this difficulty, primarily by faith and then by using the illustrations mentioned above, I mean the image and the radiance, fountain and river, essence and expression. As the Son is in the Spirit as in his own image, so also the Father is in the Son. For divine Scripture, by way of relieving the impossibility of explaining and apprehending these matters in words, has given us illustrations of this kind; that it may be lawful, because of the unbelief of presumptuous men, to speak more plainly, and to speak without danger, and to think legitimately, and to believe that there is one sanctification, which is derived from the Father, through the Son, in the Holy Spirit.
As the Son is an only-begotten offspring, so also the Spirit, being given and sent from the Son, is himself one and not many, nor one from among many, but Only Spirit. As the Son, the living Word, is one, so must the vital activity and gift whereby he sanctifies and enlightens be one perfect and complete; which is said to proceed from the Father, because it is from the Word, who is confessed to be from the Father, that it shines forth and is sent and is given. The Son is sent from the Father; for he says, "God so loved the world that he gave his only begotten Son." (Joh 3:16) The Son sends the Spirit ; "If I go away, I will send the Paraclete." (Joh 16:7) The Son glorifies the Father, saying: "Father, I have glorified thee." (Joh 17:4) The Spirit glorifies the Son; for he says: "He shall glorify me." (Joh 16:14) The Son says: "The things I heard from the Father speak I unto the world." (Joh 15:15) The Spirit takes of the Son ; "He shall take of mine and shall declare it unto you." (Joh 16:15) The Son came in the name of the Father; "I am come in my Father's name" (Joh 5:43) The Holy Spirit came in the name of the Son,"But the Comforter, which is the Holy Spirit, whom the Father will send in my name." (Joh 14:26)
But, beyond these sayings, let us look at the very tradition, teaching, and faith of the Catholic Church from the beginning, which the Lord gave, the Apostles preached, and the Fathers kept. Upon this the Church is founded, and he who should fall away from it would not be a Christian, and should no longer be so called. There is, then, a Trinity, holy and complete, confessed to be God in Father, Son, and Holy Spirit, having nothing foreign or external mixed with it, not composed of one that creates and one that is originated, but all creative ; and it is consistent and in nature indivisible, and its activity is one. The Father does all things through the Word in the Holy Spirit. Thus the unity of the holy Trinity is preserved. Thus one God is preached in the Church, "who is over all, and through all, and in all" (Eph 4:6) "Over all", as Father, as beginning, as fountain; "through all", through the Word; "in all", in the Holy Spirit. It is a Trinity not only in name and form of speech, but in truth and actuality. For as the Father is he that is, so also his Word is one that is and God over all. And the Holy Spirit is not without actual existence, but exists and has true being. Less than these (Persons) the Catholic Church does not hold, lest she sink to the level of the modern Jews, imitators of Caiaphas, and to the level of Sabellius. Nor does she add to them by speculation, lest she be carried into the polytheism of the heathen. And that they may know this to be the faith of the Church, let them learn how the Lord, when sending forth the Apostles, ordered them to lay this foundation for the Church, saying: "Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit." (Mt 28:19) The Apostles went, and thus they taught; and this is the preaching that extends to the whole Church which is under heaven.
Saint Athanasius: Letters to Serapion, Letter I, 18-28