The great enigma
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Why is there one single God in three persons? God exists of himself, a se in Latin, and that is why this faculty is named aseity: it bears out the existence of a trinitarian God.
Aseity is the special attribute of God. When Moses questioned him about his name, God replied: "I am who I am" (Ex 3: 14) and later gave his name on several occasions as being "the Eternal", saying "This is my name forever". But what is eternity?
In the context of religion, eternity cannot be limited by any chronological definition. St Thomas Aquinas in his Summa makes a distinction between Eternity, which has neither a beginning nor an end; spiritual time known as aevum, which has a beginning but no end; and earthly time, which has a beginning and will have an end. But to say that eternity has neither a beginning nor an end is an insufficient and negative definition.
On the other hand, God Himself defines eternity by calling himself, "I am", and then "the Eternal": both definitions apply to Him. So the Eternal is "I am", the first person of an eternal present which is God himself. God is in effect all that he has: not only does He possess eternity, but he is eternity; not only does He possess aseity, but is aseity; just as He not only has life, but is "the life" (John 14: 6). In Him, having and being are the same thing.
In the Old Testament when the Eternal calls himself "I am", the first person singular form of the verb indicates that He is one God. The New Testament adds: in three persons, thus supplying the key to our understanding of aseity.
In order to exist of himself, a se, the Eternal must at the same time beget and be begotten. God the begetter is the Father, he who engenders, "the principle without principle" (Council of Florence). God the begotten is the Son, the end of the Father's principle, from whom he proceeds.
To proceed from means to have its origin in. Thus, an end has its origins in a principle from which it proceeds. The principle expresses an active process, while the end expresses a passive process. This vocabulary is used to describe the relationship between the three divine persons.
The Father did not exist before the Son: active generation does not precede passive generation like cause preceding effect, because in God there is no succession. He does not change. He says as much Himself: "For I the Lord do not change" (Malachi 3: 6). This is why his immutability was proclaimed by Vatican I.
Although the roles of the Father and of the Son in the context of aseity seem relatively evident, the same may not be said of the role of the Holy Spirit. Why is there this third Person who, according to dogmatic teaching, proceeds from the Father and the Son?
The way in which the Holy Spirit proceeds from the Father and the Son has been the subject of much discussion. The Catholic Church now proclaims that the Holy Spirit proceeds from the Father and the Son and yet, "and the Son", Filioque in Latin, did not feature in the Nicene Creed revised in Constantinople in 381. Its later inclusion distinguished it from the Creed of the Orthodox Church. The original omission was probably deliberate. At the time, a heretical group named the Macedonians after their leader Macedonius, maintained that the Holy Spirit had been created by the Son. The best way of refuting their belief was not therefore to declare that the Holy Spirit proceeds from the Son, but that he proceeds from the Father and should be adored jointly with the Father and the Son. The Council of Florence in 1439 held that the later addition of the Filioque was "legitimate and reasonable". It would moreover seem logical, since the Son is the image of the Father - "Whoever has seen me has seen the Father" (John 14: 9) says Jesus - and since the Father is the principle of the Holy Spirit, the Son as the image of the Father must likewise be the principle of the Holy Spirit.
According to the Catholic faith, the Holy Spirit thus proceeds both from the Father and the Son, who are his principle, and of whom he is the end. This one single principle, called active spiration, is evidently not a person but the active union of the first two persons. Because his principle is active spiration, the symmetrically opposite Holy Spirit is said to be passive spiration.
Spiration comes from the Latin spiratio which means breath. The Father and the Son, partners in active spiration, are thus represented here by two opposing breaths which unite, and from which proceeds a third, the passive spiration: the Holy Spirit.
To say that the Holy Spirit proceeds from the union of the Father and the Son as from the union of two breaths is to call upon a physical image, and it is more appropriate to fall back on the definition: "God is love" (1 John 4: 8) placing it within the context of the Trinity. The Father and the Son love each other, and this mutual, enduring and necessary love is the Holy Spirit, as summarized by Catholic teaching. It is also in the sense that union is synonymous with love that the Holy Spirit is identified as being the union of the Father and the Son.
But why must this union subsist in a third person? And what is to be gained by using this fundamental term, person, in respect of God?
We must be very careful here. We are venturing into the heart of the mystery of the Blessed Trinity and we would do well to ask ourselves whether we have the right to go any further. A canon dating from the First Vatican Council states: "If any one say that in Divine Revelation there are contained no mysteries properly so called but that through reason rightly developed all the dogmas of faith can be understood and demonstrated from natural principles: let him be anathema"
It is certain that reason, even "rightly developed", would not have been able to conceive of the mystery of a single God in three persons. On the other hand, since God has revealed to mankind the names of the members of the Blessed Trinity, and given mankind words with which to speak of them, the desire we feel to describe the Blessed Trinity is legitimate. We should use an appropriate vocabulary which has been developed throughout the centuries, from Revelation, by the Church Fathers and their successors, using terms often borrowed from the field of logic or even terms which are invented, such as spiration, or adopted through analogy, as is the case with person.
The term person is found neither in the Apostles' Creed, nor in that of Constantinople. It was finally accepted, but not without many reservations, such as those expressed by Saint Augustine: "Yet, when the question is asked, What three? Human language labours altogether under great poverty of speech. The answer, however, is given, three persons, not that it might be [completely] spoken, but that it might not be left [wholly] unspoken." (De Trinitate V, 9)
It is not only through analogy with the human person that the divine persons are so called, but also because the person of the Word assumed human nature.
The divine persons are currently defined as three intratrinitarian relationships which continue to exist, each in a different way, having the same essence. It is also said that they have the same nature or the same substance. Could we go further and indeed suggest that the three persons subsist, each in a different way, in one single divine generation? Divine generation is here the action by which God is by Himself, that is, aseity.
The three intratrinitarian personal relationships may be described as follows: that of the Father in regard to the Son is one of fatherhood or active generation; the relationship of the Son in regard to the Father is filial, or passive generation; that of the Holy Spirit in regard to the Father and the Son is passive spiration or unitive generation. We will define this later. The three persons are each the action of divine generation within which each operates in a different way.
In effect, paternity, or active generation, stands in opposition to filiation, or passive generation, and so the Father is distinct from the Son. Active spiration, the active union of the Father and the Son, stands in opposition to their passive union, or passive spiration: the Holy Spirit, which is therefore distinct from the Father and the Son. The notion of opposition here is fundamental, as stated by the Council of Florence in 1442: "Everything (in them) is one where there is no opposition of relationship."
The above discussion is founded upon Revelation, which we will now use as the basis for clarifying why God cannot exist in one or two Persons.
If God consisted of only one person, this would be the Father, the Ho Theos of Origen, the principle of divinity: but as such He necessarily has an end and we come back to the procession of the Trinity…
Descartes' belief that God is his own cause and therefore created himself is not acceptable: a cause necessarily precedes an effect and therefore exists before it. This would be impossible as God could not exist before He created himself.
The Old Testament however, no doubt in order to avoid the temptation of polytheism, does not speak in clear terms of the divine persons. Sketchy portrayals of the Trinity, such as the three men who appeared to Abraham (Gen 18), are not convincing: "tres vidit et unum adoravit" (he saw three and adored one) says the Roman Breviary. We can therefore understand the incredulity of Jesus' contemporaries who adhered strictly to the Holy Book in which God, using the first person singular, had said to Moses: "I am who I am". This incredulity was later stretched even further by the difficulty of the mystery of one God in three persons: the cornerstone of the Christian faith. Beneath the shadow of this one God the veiled form of the Holy Trinity is discernible.
The New Testament revealed the existence of the Father and the Son, so why does God not exist only in these two persons: the begetter and the begotten, the active generation and the passive generation?
God cannot exist in two persons. Because of the distinction between the two, and the fact that they stand in opposition and are therefore separate, the Father and the Son, the active generation and the passive generation, could not continue to exist without the participation of a third relation. This third relation is the unitive generation, the Holy Spirit, which acts simultaneously with the first two to unite them.
The procession of the Holy Spirit might seem difficult to accept, for it is natural to think that the third person, evidently following the first two, was not present during the generation of the Son by the Father, and would seem to precede his own procession.
It is, however, essential to remember that because of divine immutability, which is an article of faith, there is no succession in God, and that the three persons subsist simultaneously. So the Holy Spirit, although proceeding from the Father and the Son, subsists with them and acts in them.
We can draw an analogy with a simple line, L, which because of its two opposing directions provides a very simplified diagram of the procession of the Trinity. The Father is represented by a line L along the positive axis, the Son is represented by the same line L along the negative axis, and the Holy Spirit by the same line L which has neither a positive nor a negative value. L+ and L- are equal and opposite and therefore distinct. The line without a numerical value is equal to the first two because it is the same line; it is distinct because it has neither a positive nor a negative value.
By the very fact of its existence, the line L+ gives rise to its opposite L-, which is its mirror image, as the Father gives rise to the Son in his image. The sum of (L+) and (L-) corresponds to the active union, called active spiration, of the first two persons.
This sum, which is an analogy of active spiration, is as such the origin of the analogy of the passive union, called passive spiration: the Holy Spirit. This second analogy is the line L bearing no numerical value because it proceeds from the union of the opposites L+ and L- in which the positive and negative values are neutralised, resulting in L. Without this unplotted line, L+ and L- could not subsist because they could not be united.
In the same way that there is one God in three persons, in this analogy there is one single line indicating three relationships which, if we pursue the analogy of the Trinity, are each in each of the other two.
Our Faith tells us that if the three persons consist of divine Substance, each of them is necessarily in each of the two others: the Father is in the Son and the Son in the Father; the Father and the Son are in the Holy Spirit and the Holy Spirit is in both of them. This dogmatic teaching, partially revealed by Jesus: "Do you not believe that I am in the Father and the Father is in me?" (John 14: 10), was explicitly formulated by the Council of Florence and bears the name of circumincession.
Aseity is God. He could not exist if He did not beget himself in the active generation of the Father; He could not exist if he were not begotten in the passive generation of the Son; He could not exist if the Father and the Son were not united in the unitive generation of the Holy Spirit. God is therefore of necessity in three persons.
There is only one aim behind all this activity: that God may eternally manifest aseity and, in the traditional formula, thus live forever and ever.