SUMMA THEOLOGICA
Q 27: THE PROCESSION OF THE DIVINE PERSONS, A1: Whether there is procession (motion) in God?
The Most Holy Trinity

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." 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." CCC 234
 
This Mystery stems forth directly from Sacred Scripture and Sacred Tradition. St. Thomas Aquinas points out that Sacred Scripture uses words in relation to God which signify motion, for instance, Our Lord says, "From God I came forth (proceeded)" (Jn1:14; 6:42-46; 8:42; 13:3; 15:26; 16:27-28). This motion of coming forth (proceeding) has been understood differently by people.
 
Some have understood this motion in the sense of an external thing coming from its maker; so Arius, a priest of the fourth century took it, saying that The Son comes forth from The Father as His first and greatest creature, and that The Holy Spirit comes forth from The Father and The Son as the creature of both! If this were true, however, neither the Son nor the Holy Spirit would be True God: and this is contrary to what is said of The Son, "That. . .we may be in His true Son. This (Jesus) is True God" (I Jn 5:20).  Of the Holy Spirit too it is also said, "Know you not that your bodies are the temple of the Holy Spirit?" (I Cor 6:19). Now, to have a temple is God's prerogative. Therefore, the Holy Spirit IS God, and NOT a creature from God.
 
Still others take this coming forth of The Son from The Father (procession) to mean a thing causing a certain material effect, as impressing its own likeness upon another, like an image stamped on paper. They say that God The Father is called “Son” in taking on flesh from the Virgin Mary, and that The Father is also called “The Holy Spirit” in making man holy, and moving him to divine life. However, the words of Our Lord contradict such a meaning, when He speaks of Himself, "The Son cannot do anything of Himself” (Jn 5:19); while many other passages show the same, whereby we know that The Father is not The Son.
 
The Catholic Position
Careful examination shows that both of these opinions (above) take motion as meaning an outward material act; hence neither of them affirms motion as existing interiorly and spiritually in God Himself; but, since motion always means action, and because there is an outward external motion of external matter, so there must be an inward motion corresponding to the internal action remaining within the intellectual agent: an imminent kind of thinking and willing to act. Man is made in God’s Image and Likeness, so we can see in the effect (man) something of the cause (God).
               
This kind of internal action does indeed take place in the thinking process of man. For example, when we come to understand a new thing, our very of process of understanding (thinking) means there is an interior motion within us. This interior motion within us is called a ‘conception’ (a giving birth) within our mind of the object understood, a concept issuing forth, being conceived, from our intellectual power and proceeding from our understanding of that object (This is often depicted as a light bulb going on in the mind of a cartoon character). Hence, when the thinker (man) ‘thinks’, he conceives a new concept, a new conception, which is in turn loved by the thinker. For example, we all love to think about ice cream in the summer, especially thinking about eating ice cream, thus we bring this internally loved concept of our mind (eating ice cream) into external action: we go to Dairy Queen, etc.!  Notice that what began as the introduction to the mind of a new reality, “ice cream”, the concept was born in the interior of the mind, conceived by the intellect-mind (thinker), then in turn loved by the 

free-will, and it is finally acted upon by the free-will bringing it into external reality for one’s self, usually first by the word, “Let’s go to …” (This is the very nature of advertisements). And because the human person’s intellectual ‘concept’ is usually first made external by the spoken word; it is called the ‘word of the heart (will)’, signified by the word of the voice.
 
What we are seeing is that, as God is above all things, we should therefore understand what is said of God, not according to the mode of the lowest earthly creatures, namely external motions of physical bodies, but from a likeness of the highest creatures, the intellectual substances; while even the likeness’s derived from these falls short in the understanding of the Divine Reality Who is God. Thus, in the example, God The Father is ‘The Thinker’, God The Son is ‘The Thought’ or Concept, and God The Holy Spirit is ‘The Love’ that The Father has for The Son-Concept, AND is also -- at one and the same time -- The Love that God The Son has for God The Father. We can understand then that God The Father’s intellectual ‘concept’ remains internal, however, it is named after the corporeal spoken word that man understands; hence it is called “The Word”. In this way we understand St. John’s Gospel [Jn 1:1–4,14] introduction of God The Father from all eternity, with The Son, The Word of God…
 
In the beginning was The Word, and The Word was with God, and The Word was God. He was in the beginning with God; all things were made through Him, and without Him was not anything made that was made. In Him was life, and this life was the light of men.  . . . 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.
 
Of course, unlike us, God The Father never began in time, He never ‘started’ to begin to think of The Son Who never began in time either; rather, The Son was present from all eternity with The Father, bringing forth (spirating/spiration) the Love that is God The Holy Spirit. Hence The Son is said to be “Eternally Begotten” of The Father, for it never was that He was not.
 
Conclusion 
Motion within the Godhead is not, therefore, to be understood from what it is in material bodies, either according to local movement or by way of a cause proceeding forth to its exterior effect, as, for instance, like heat from the agent to the thing coming to be made hot. Rather it is to be understood by way of an interior intelligible emanation, for example, of the intelligible word which proceeds from the speaker, yet remains within him. In this sense the Catholic Faith understands procession (motion) as existing within The Godhead. The Nicene Creed was formulated in the year 325 by the assembly of all the Christian (Catholic) Bishops of the world. They made the Creed the standard of what we as Christians are to believe, and we, as Catholics, have held to that same Creed since that time.
 
I believe in One God, The Father, the Almighty. . .
I believe in One Lord, Jesus Christ, The Only Son of God, Eternally Begotten of The Father . . .
I believe in The Holy Spirit, The Lord, the giver of life, Who proceeds from The Father and The Son.
With The Father and The Son He is worshipped and glorified. . . .