According to Catholic dogma, there are two inherent, eternal, and substantial processes in God: one by which the second divine person originates from the first as the Son from the Father, which therefore bears the name generation, begetting, birth (generatio), and one by which the Holy Spirit proceeds from the Father and the Son as from one principle, which we call spiration (spiratio).
A procession (processio, ἐκπόρευσις) generally refers to a process that starts from one reality and ends at another, with the content being the real existence of the endpoint originating from the starting point. Since God is simple, in God there can only be immanent processions: the starting and endpoint of the procession cannot exceed the boundaries of the divine reality, but remains entirely within the Godhead. And since the Godhead itself cannot be divided in its essence, the resulting reality can only encompass the entire, undivided divine reality; in other words, divine procession can only be substantial, that is, the resulting reality can only be God. Finally, since God is pure actuality (actus purus), processions in God cannot represent transitions from potentiality to actuality; that is, they are eternal. Therefore, the originator cannot exert a creative, constitutive, or causative activity in relation to the originated; hence, the originating persons cannot be called the cause of the originated but only their principle (principium); the principle is a more general concept than cause and does not necessarily express a causal relationship; for example, the point is the principle of the line, but not its cause.
In the Prologue to the Epistle to the Hebrews (1:5), the divinity of Christ is proclaimed, referring to these words of the second psalm: "You are my Son, today I have begotten you." Christ says that the Holy Spirit proceeds from the Father (Jn 15:26). Based on such biblical revelations, theology talks about the Trinitarian origins or derivations (processiones trinitariae), and about two forms of origins. Theologians refer to the origin of the Son as generation or birth (generatio), and that of the Holy Spirit as simple origin or derivation (processio simpliciter).
Since each person of the Trinity is God, and God has existed eternally, it is utterly impossible for any divine person to precede the others in time; and since the persons share a single nature, no hierarchical difference can arise among them. Therefore, the Trinitarian origins merely signify logical succession, that is, the logical rationale (ratio) and principle basis (principium) of one person are different from those of another. The Father is a Father by constantly transmitting his essence to the Son from eternity, and the Son is a Son by eternally accepting this essence, in which the essence of the Father is eternally reflected. This acceptance, this becoming of the person in reflection, is what we call sonship or birth. The divine essence is the same in all three persons, but the "mode of existence" of this essence is quite different in the Father, who eternally imbues it into the Son, and in the Son, who eternally accepts it, and in the Holy Spirit, to whom the Father and the Son also eternally transmit it, and who accepts it from them.
The earliest church fathers tried to illustrate the birth of the Son with analogies. Just as the rays of the sun constantly emanate from the sun as long as it exists, and just as water constantly trickles from an inexhaustible spring as long as the spring exists; the Son is similarly constantly born from the Father, indeed eternally, since the Father exists eternally. The "today" in the second Psalm refers to God's "eternal present," as there is no past and future, yesterday and tomorrow, for Him - as the dogma about God's eternity teaches. The Trinitarian origins are eternal origins. The church fathers' analogies are only partly accurate, and can be critiqued from multiple perspectives. For instance, the sun is the physical cause of the rays, the spring is of the brook, but physical causality must be excluded from God: God is not a cause of Himself (causa sui), but the spiritual rationale of His being (ratio sui), which means that the rationale of His existence is not to be found outside Him, but within Him. Nor are the aforementioned analogies good because the spring's water would be more if it didn't flow out as a brook, but the divine essence cannot lose anything in the Trinitarian origins, neither with the birth of the Son, nor with the origin of the Holy Spirit; since God is absolutely unchanging and indivisible. Due to the same unchangeability, neither the Son can gain anything extra by being born, nor the Holy Spirit by originating. The birth of the Son and the origin of the Holy Spirit - as stated above - may only create a different mode of existence for the same divine essence, but by no means a change. Because of this identity of essence, classical Trinitarianism calls the Trinitarian origins substantial origins (processiones substantiales). The latter also means that not only the originator but also the originated is God, as these origins are the various forms of existence of the common divine essence (substantia).
According to Augustine, since God is spirit, we should look for analogies in the realm of spiritual processes when we want to study God's inner life.
One of the most important manifestations of our spiritual life is the formation of concepts, the birth of our notions. Just as an unexpressed concept (verbum mentis) is conceived, born in our consciousness, the Son is born from the Father in the same way. The Son is essentially nothing more than the concept that the Father forms of himself, his self-knowledge, which on the one hand has always been there, and on the other hand possesses such power, intensity that it becomes a separate person.
Since God is the infinite perfection of all values (true, good, beautiful, holy), and these values provoke spiritual love from the soul, the Father also infinitely loves himself as a totality of value, and this infinite love must also be reflected in the Son. The love of the Father reflected in the Son and the reflection of this love in the Father, as if “bouncing back”, is essentially one and eternal love, and it is also so intense that it becomes a separate person, the person of the Holy Spirit.
Augustine's analogy has three advantages:
a) It aptly shows that we can rightly speak of a spiritual kind of birth and origin, such as we encounter with the persons of the Trinity.
b) What is born in the human soul as self-knowledge, and what is created as love, can also be "immanent": the originated does not "step out" of the originator in this case, just as the spoken word "steps out" of the speaker, or as the born child essentially separates from its mother. This immanence characterizes the Trinitarian origins: the life of the Son, indeed his entire essence, is identical with that of the Father, he does not step out of him, he does not separate from him in any reality; similarly, the life and essence of the Holy Spirit remains the same with the other two persons and stays within them.
c) The divine and human self-knowledge are similar in that the birth of both can be equally referred to as conception and birth. Since the conception and birth of our thoughts coincide in time, not as it happens in the birth and conception of animals and humans. That's why these two expressions are completely synonymous: "the Father has been begetting the Son from eternity", and "the Father has been giving birth to the Son from eternity".
Of course, there are essential differences between divine and human self-knowledge and self-love. These were already noticed by Augustine. Divine and human self-knowledge and self-love primarily differ from each other in that our conscious self-knowledge and self-love are not present from the first moment of our existence. Another difference is that human self-knowledge gradually unfolds, and even so, it does not become entirely perfect; the same goes for our self-love. Our internal image formed about ourselves never fully reflects what we are. However, the Father, without residue, perfectly "speaks into" his entire essence to the Son and loves him in the Holy Spirit. The third and most important difference is this: The intensity, the power of God's self-knowledge and the mutual love of the first two persons of the Trinity is such that this knowledge and love transcends the realm of thought (ens rationis) and enters the realm of reality (ens reale): it becomes a separate divine person that really exists, although its immanence also remains.
We assert that the Father is without origin and birth (principium sine principio).
Indeed, the Scripture attributes origin to the other two persons, but never to the Father, and by this, it implicitly teaches his lack of origin. Early patristics applied such descriptors to the Father: without beginning (anarchos, ἄναρχος, ἀναίτιος), uncreated (agenetos, ἀγένητος), unbegotten (agennetos, ἀγέννητος), there is no principle from which he would originate. This statement, however, is linguistically negative in meaning, yet it proclaims the positive that the Father possesses the common divine essence in such a way that He does not receive it from anyone, but only gives it to the Son and, together with the Son, to the Holy Spirit. The Father is "principium sine principio". He is the ultimate solution to the origin of the other two persons.
Paul considers the fatherhood of God so important that – as we have already seen – instead of Father, he sometimes says God. He is primarily the "Father of our Lord Jesus Christ" (2 Cor 1:3), to whom Jesus turns not only as a human, but also as the second divine person, with feelings of devotion and mutual love; and as the God-man, he comes into the world as the Father's emissary (John 3:17), emptying himself (Phil 2:7) to reconcile the world with the Father and make humans God's children. His entire human life is childlike devotion before the heavenly Father, from whom he received his divine essence, and whom in this sense he can call greater than himself (John 14:28). His perfect self-sacrifice is both a model and a means for humans to become children of the heavenly Father in a non-identical, but analogous sense. Because God also wants to be primarily a Father to us: "For whom he foreknew, he also predestinated to be made conformable to the image of his Son; that he might be the firstborn amongst many brethren" (Romans 8:29; cf. Galatians 3:26). The characteristic warmth of the New Testament is that God spoke the final word to humanity as Father. He sent his Son and revealed through him that he accepts humanity into his merciful love.
The Son originates from the Father through generation, begetting, birth.
This follows from the fact that the second person is a son to the Father not only in a moral sense, but also in a metaphysical sense. According to the Scriptures,
a) the second divine person is the only-begotten Son of the Father in a natural sense, in the metaphysical sense of the word (φύσει not θέσει, that is, not by adoption). However, the natural son originates from the father by generation.
b) This is also formally taught in the New Testament, when it correctly interprets the places in the Old Testament that refer to the Son's birth. So: "To which of the angels did He ever say (the Father): 'You are my Son, today I have begotten you'?" (Heb 1:5–Ps 2.) "The seas were not yet, and I was already conceived." (Prov 8:24) Furthermore: "No one has ever seen God; the only-begotten Son, who is in the bosom of the Father, has declared him." (Jn 1:18; cf. 1,1.)
c) The Father is the primal model and source of all fatherhood: "For this cause I bow my knees to the Father of our Lord Jesus Christ, of whom all paternity [i.e. fatherhood] in heaven and earth is named." (Eph 3:14.) Similarly, the second divine person is the primal model and form of all sonship: "For those whom he foreknew, he also predestined to be conformed to the image of his Son, that he might be the firstborn among many brethren." (Rom 8:29 cf. Gal 3:26) However, sonship is characterized by origination through generation. If this were missing in the Son, it would be a false or distorted primal model and pattern.
Sonship in the natural, metaphysical sense necessarily presupposes generation or being generated. Therefore, he is the "own" Son (Rom 8:3), the "only-begotten" Son (John 1:14; 1 John 4:9) of the Father. Following the letter to the Hebrews (1:5), we can also refer to two expressions of the Psalms: to give birth (110:3), to give life (2:7).
This is also the universal teaching of the Church Fathers even before the Council of Nicaea. According to Justin, the Logos is that God whom the Father has begotten (Justin. I 61 62). The teachings of the Apologists are somewhat obscured by their less successful attempts to associate the birth of the eternal Word with the creation of the world (Clem. Al. Adumbr. (Μ 9, 734), Origen. in Jer hom. 9, 4). The Alexandrians and Tertullian are more precise in this regard (Tertull. Prax 2 8 9; Marc. II 27.). Later, in opposition to the Arians, especially the Greek Church Fathers defended the eternity of the Son's generation (the Arians' main argument was that the begotten is later than the begetter, see: Nyssen. Eunom. (M 45, 441 ff.); Basil. Eunom. II 17; cf. Thom I 42, 2.); also the necessity of this generation (according to the Arians, God could not be forced to beget, so the Son exists by the will of the Father, that is, he was created), which, however, is as different from blind compulsion as the free decision to create (See Athanas. Ctra Ar. or. 3, 60 ff. cf. 1, 21–28; Nazianz. Or 29, 1.); and finally, its substantial and spiritual nature (according to the Arians, generation involves division), which is compatible with God's absolute simplicity (Athanas. Decret. Nicean. 13; Nyssen. Eunom. IV. (M 45, 617 ff.); Cyrill. A. Thesaur. 6.).
We must understand the Son's generation or birth based on the pattern of earthly children's generation, but not in an identical, but in an analogous sense. For God is spirit, so only a spiritual birth can occur with him. However, the analogy is maintained, so we must speak of a real birth. Because here everything is realized that is included in the definition of earthly birth: the living comes from the living, the two have a living connection, and the origin implies essential identity.
When we say "verbum mentis" with Augustine, we emphasize the immanence of the Son. However, when we see birth in the Son's origin, we do not emphasize immanence, but the communication, the "handing over" of identical nature.
The sonship of the second person is also of great significance in the order of salvation. According to Paul, the Father created everything in him that is in heaven and on earth, and everything subsists in him (Col 1:16-17). Even before the creation of the world, the Father chose the called ones in him (Eph 1:4). For the Father constantly speaks his eternal thoughts into the Son, so the Son could be the Father's measure in the creation of the world. Therefore, he is the founder of God's kingdom, he is the norm of all moral perfection, therefore he will be the measure and executor of the last judgement. The final state is formed in such a way that the Father brings together all created values under his sovereignty (Eph 1:10). The Son is also the "causa formalis" of our individual supernatural life, insofar as the grace, the giving of which is the common work of the three divine persons, transfers the image of sonship to the justified human soul. We will become children of the Father in the same form as Christ is in childlike relation to the heavenly Father. Thus we become partakers of Christ's divine sonship and become co-heirs with him. And just as the Spirit connects Christ with the Father, the Holy Spirit will also be the soul of our childlike relationship.