What is a mystery? In Theology, the term "mystery" refers to truths revealed by God that surpass the powers of natural reason; mysteries are truths that human reason cannot reach without the aid of faith which accepts what God has told us.
Is "mystery" the same as "something we cannot know"? No. There are many things which our minds can understand or know but are nevertheless unknown to us--they are "accidentally" unknowable--for different reasons: things of the future, things that are remote to us, or are hidden from us.
Is "mystery" the same as "something we cannot grasp or comprehend"? In theological language, no. Because we cannot completely comprehend even the things that we know with our reason alone (unaided by faith). Even to our days, we still have not exhaustively understood the simplest non-living elements that make up the material world. Science continues to reveal "mysteries" about the subatomic particles, genetic material, world climate, and so on.
1.1 What the mystery is all about
What's so mysterious about it? We believe that there is only one God. We believe that the Father is God, the Son is God, and the Holy Spirit is God. But the Father is neither the Son nor the Holy Spirit, the Son is neither the Father nor the Holy Spirit, and the Holy Spirit is neither the Son nor the Father--they mutually exclude each other. Nonetheless, there is only one God and not three. Therein lies the mystery.
Why should that be a mystery? Let us take three men: Joe, John and James. Joe is a person, John is a person, Joe is a person. That makes three persons. That also makes three men. In the case of the Most Blessed Trinity it is not the case. The Father is God, the Son is God, the Holy Spirit is God. That's three Persons. But the three Persons are not three Gods. This is the mystery, the truth, that we will not be able to know if God did not tell us that it were so.
1.2 How important is this mystery to us believers?
Here is what no 234 of the CCC teaches:
The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the "hierarchy of the truths of faith".[General Catechetical Directory, 43] The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men "and reconciles and unites with himself those who turn away from sin".[General Catechetical Directory, 47]
There are two points to consider here:
With respect to the involvement of the Blessed Trinity in our salvation and sanctification, we could use an analogy. Our coming to form part of the household of God in heaven is like one big project. That we be transformed from sinners to saints is, in fact, the most ambitious project ever launched in the whole history of mankind. In this big project, each Person has a role to play.
The practical consequence of this is that here on earth we need to deal with each of the three Divine Persons if we are to reach what Pope Benedict XVI calls "the supreme moment of satisfaction ... in which we are simply overwhelmed with joy" (Spe Salvi, 12).
2.1 Old Testament texts on the Trinity
There is no clear revelation in the Old Testament of the Blessed Trinity. A proof of this is that present-day Judaism does not seem to recognise the Trinity. There are some hints of the three Persons, however, that we can find in the Old Testament. We can see these in passages that speak about the Son of God. In Psalm 2:7, we read: “You are my Son, I have begotten you this day”. There are also passages about the Divine Wisdom, the Word of God, and his Spirit (cf Proverbs 8:22‑31; Wisdom 7:25‑27; 9:17).
We can find other indirect references to the Trinity in the use of majestic plural (“Let us make man to our image and likeness...” Genesis 1:26, cf Genesis 3:22 and 11:7), and the use in Hebrew of the plural form for God's name, Elohim “the Most High,” but with singular meaning.
2.2 New Testament texts on the Trinity
It was Jesus who taught us in clear terms about the Blessed Trinity. GH Joyce writes in the Catholic Encyclopedia:
First He taught them to recognize in Himself the Eternal Son of God. When His ministry was drawing to a close, He promised that the Father would send another Divine Person, the Holy Spirit, in His place. Finally after His resurrection, He revealed the doctrine in explicit terms, bidding them "go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost" (Matthew 28:18). The force of this passage is decisive. That "the Father" and "the Son" are distinct Persons follows from the terms themselves, which are mutually exclusive. The mention of the Holy Spirit in the same series, the names being connected one with the other by the conjunctions "and . . . and" is evidence that we have here a Third Person co-ordinate with the Father and the Son, and excludes altogether the supposition that the Apostles understood the Holy Spirit not as a distinct Person, but as God viewed in His action on creatures.
There are many texts that refer to each of the Persons individually. There are also texts that refer to the three of them together, such as the following:
2.3 The Trinity in the Creed
All the Professions of Faith from the beginning of Christianity to our times profess belief in three Persons and one God.
"I believe in God, the Father Almighty ... I believe in Jesus Christ ... I believe in the Holy Spirit ...."
We find the Trinity also in the most basic prayers such as the sign of the Cross and the Glory be.
"In the name of the Father, and of the Son, and of the Holy Spirit. Amen." (Note that we do not say "names", but name. This is to show that we pray in the name of only one God.)
"Glory be to the Father, and to the Son, and to the Holy Spirit. As it was in the beginning, is now, and ever shall be, world without end. Amen."
One of the most complete creeds that are extremely helpful to our discussion is the Athanasian Creed that dates back to the 4th or 5th century. [Click here for the text of the Athanasian Creed.]
3.1 Scriptural bases for philosophical discussion
The most important reference for a philosophical discussion of the Blessed Trinity is found in Saint Thomas Aquinas' Summa Theologiae, part I, questions 27 to 43.
The term procession is taken by Saint Thomas from John 8:42, where Jesus Christ says, "I proceeded and came forth from God." (Cf Summa Theologiae, part I, question 27, article 1)
The term generation is also applied to the Son, and Saint Thomas uses the words of Psalm 2:7: "This day have I begotten Thee." (Cf Summa Theologiae, part I, question 27, article 2)
There is another procession aside from generation. Saint Thomas writes in the Summa Theologiae (part I, question 27, article 3):
The Holy Spirit proceeds from the Father (John 15:26); and He is distinct from the Son, according to the words, "I will ask My Father, and He will give you another Paraclete" (John 14:16). Therefore in God another procession exists besides the procession of the Word.
He then argues that the first procession (that of the Son) is by way of knowledge, there is a second procession (that of the Holy Spirit) by way of love:
The procession of the Word is by way of an intelligible operation. The operation of the will within ourselves involves also another procession, that of love, whereby the object loved is in the lover; as, by the conception of the word, the object spoken of or understood is in the intelligent agent. Hence, besides the procession of the Word in God, there exists in Him another procession called the procession of love.
We shall see this more in detail below.
3.2 Processions through knowledge and love
How does the Son proceed from the Father by an intellectual operation?
In order to understand the procession of the Son from the Father, it is important to understand how we humans know. When someone explains something that is new to us, he may ask us at the end of his explanation whether we are able to form an idea or a concept of what he is saying. The idea or concept is in our mind, and it is a likeness of the thing that is being explained to us.
Just as we can know things around us, with time we also know ourselves--we start forming an idea of ourselves. That idea in our mind is, hopefully, a likeness of our real self. It is further enriched with time through our own experience and through feedback from others. Saint Thomas Aquinas also calls this idea or interior concept of the mind the "word". It is one of the three meanings of "word" in Saint Thomas (see Summa Theologiae, part 1, question 34, article 1). To avoid confusion, we can use the term "mental word" to refer to the idea.
Now, God (from whom we get our intellect) also understands Himself; He has an Idea of Himself. Unlike us, who have only an imperfect idea of ourselves, God is able to understand Himself fully. The Idea or Concept that results from this is what we call the "Word." The Word is as perfect as the Father is. He is as true, as good, as beautiful, as eternal as the Father is. The Word is God.
This is why Saint John, in the first chapter of his Gospel, tells us:
1 In the beginning was the Word,
and the Word was with God,
and the Word was God.
2 He was in the beginning with God;
3 all things were made through him,
and without him was not anything made that was made. ...
14 And the Word became flesh and dwelt among us,
full of grace and truth;
we have beheld his glory,
glory as of the only Son from the Father.
How does the Holy Spirit proceed from the Father and the Son by a procession of love?
Aside from our intellect, we also have a will. Once we perceive something as good, we are attracted to it, we start to love it. In the case of God, we have seen above that the Son is the perfect reflection of the Father. The Father's infinite goodness is reflected thus in the Son. Love ensues. Saint Thomas quotes Saint Augustine here: "The Holy Spirit is He whereby the Begotten is loved by the one begetting and loves His Begetter (De Trinitate, vi, 5)." (See Summa Theologiae, part 1, question 37). The Holy Spirit is the Love between the Father and the Son.
3.3 Two Processions, Four Relations and Three Persons in God
In order to use our mind to understand the Blessed Trinity, it is important to have a good grasp of the terms that we use. These terms have been clearly defined by Saint Thomas Aquinas (1225-1274).The Catechism of the Catholic Church teaches:
251 In order to articulate the dogma of the Trinity, the Church had to develop her own terminology with the help of certain notions of philosophical origin: "substance", "person" or "hypostasis", "relation" and so on. In doing this, she did not submit the faith to human wisdom, but gave a new and unprecedented meaning to these terms, which from then on would be used to signify an ineffable mystery, "infinitely beyond all that we can humanly understand" [Paul VI, Solemn Profession of Faith: Credo of the People of God, no 2.]
252 The Church uses (I) the term "substance" (rendered also at times by "essence" or "nature") to designate the divine being in its unity, (II) the term "person" or "hypostasis" to designate the Father, Son and Holy Spirit in the real distinction among them, and (III) the term "relation" to designate the fact that their distinction lies in the relationship of each to the others.
Thus, the CCC (no 255) states:
The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationships which relate them to one another: "In the relational names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. ... Indeed "everything (in them) is one where there is no opposition of relationship."[Council of Florence (1442): DS 1330] "Because of that unity the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son."[Council of Florence (1442): DS 1331]
The word "mission" comes from a Latin word which means "to send". The destination of this mission are those who are in sanctifying grace. Saint Thomas Aquinas explains (Summa Theologiae, part I, question 43, article 6):
According to Augustine (De Trinitate, iii, 4; xv, 27), the invisible mission is for the creature's sanctification. Now every creature that has grace is sanctified. Therefore the invisible mission is to every such creature.
We know that God is present in all His creatures by the mere fact of creating them. But mission is a new kind of presence of God in the soul of a person in grace.
Mission follows procession: the Son proceeds from the Father, and is sent to men by the Father; the Holy Spirit proceeds from the Father and the Son, and is sent to men by the Father and the Son. Careful study of the Scripture reveals to us that the Son is sent by the Father and the Holy Spirit is sent by the Father and the Son.
How about the Father? Is the Father sent? Saint Augustine, again quoted by Saint Thomas (Summa Theologiae, part I, question 43, article 4), says (De Trin. ii, 3), "The Father alone is never described as being sent." (De Trinitate, ii, 3)
The Catechism of the Catholic Church (no 253) teaches us:
The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity".[Council of Constantinople II (553): DS 421] The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God."[Council of Toledo XI (675): DS 530:26] In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature."[ Lateran Council IV (1215): DS 804]
Furthermore, the CCC (no 258) also teaches us:
258 The whole divine economy is the common work of the three divine persons. For as the Trinity has only one and the same natures so too does it have only one and the same operation: "The Father, the Son and the Holy Spirit are not three principles of creation but one principle."[Council of Florence (1442): DS 1331; cf. Council of Constantinople II (553): DS 421] However, each divine person performs the common work according to his unique personal property. Thus the Church confesses, following the New Testament, "one God and Father from whom all things are, and one Lord Jesus Christ, through whom all things are, and one Holy Spirit in whom all things are".[Council of Constantinople II: DS 421] It is above all the divine missions of the Son's Incarnation and the gift of the Holy Spirit that show forth the properties of the divine persons.
And in no 267, the CCC reiterates this point:
Inseparable in what they are, the Divine Persons are also inseparable in what they do. But within the single divine operation each shows forth what is proper to him in the Trinity, especially in the divine missions of the Son's Incarnation and the gift of the Holy Spirit.
What then is a Christian to do after having considered this mystery? The Catechism of the Catholic Church tells us in point 259 and 260:
259 Being a work at once common and personal, the whole divine economy makes known both what is proper to the divine persons, and their one divine nature. Hence the whole Christian life is a communion with each of the divine persons, without in any way separating them. Everyone who glorifies the Father does so through the Son in the Holy Spirit; everyone who follows Christ does so because the Father draws him and the Spirit moves him.[Cf John 6:44; Romans 8:14]
260 The ultimate end of the whole divine economy is the entry of God's creatures into the perfect unity of the Blessed Trinity.[Cf John 17:21-23] But even now we are called to be a dwelling for the Most Holy Trinity: "If a man loves me", says the Lord, "he will keep my word, and my Father will love him, and we will come to him, and make our home with him":[John 14:23]
O my God, Trinity whom I adore, help me forget myself entirely so to establish myself in you, unmovable and peaceful as if my soul were already in eternity. May nothing be able to trouble my peace or make me leave you, O my unchanging God, but may each minute bring me more deeply into your mystery! Grant my soul peace. Make it your heaven, your beloved dwelling and the place of your rest. May I never abandon you there, but may I be there, whole and entire, completely vigilant in my faith, entirely adoring, and wholly given over to your creative action.[Prayer of Blessed Elizabeth of the Trinity]
What consequence does this have for us, say, when we assist in the Holy Sacrifice of the Altar? Saint Josemaría explains:
As you attend Mass, you will learn to deepen your friendship with each one of the three divine Persons: the Father who begets the Son; the Son, begotten by the Father; the Holy Spirit, who proceeds from the Father and the Son. When we approach any one of the divine Persons, we approach the one God. And when we come close to all three Persons — Father, Son and Holy Spirit — again we come into the presence of the one true God. Love the Mass, my children, love the Mass. And be hungry to receive our Lord in communion, although you may be cold inside, although your emotions may not correspond to your desires. Receive communion with faith, with hope, with burning charity. (St Josemaría Escrivá, "The Eucharist, Mystery of Faith and Love," in Christ is Passing By, no 91)
He also explains how we discover new horizons in our prayer.
Our heart now needs to distinguish and adore each one of the divine Persons. The soul is, as it were, making a discovery in the supernatural life, like a little child opening his eyes to the world about him. The soul spends time lovingly with the Father and the Son and the Holy Spirit, and readily submits to the work of the life-giving Paraclete, who gives himself to us with no merit on our part, bestowing his gifts and the supernatural virtues! (St Josemaría Escrivá, "Towards Holiness," in Friends of God, no 306)
Glory be to the Father, to the Son, and to the Holy Spirit. As it was in the beginning, is now, and ever shall be, world without end. Amen!