Here’s a nice Wikipedia article if you would like the background information about the controversy. Today’s article explains why the Holy Spirit must proceed from Father and Son, not from the Father only.
God is three Persons but one Nature. In the divine nature, God almighty is thoroughly One. The Trinity is not three equal Gods, but one God who is Three Persons. Each of the Three Persons is entirely equal as God. For there are not three Divine Natures, which we would then have to evaluate to see if they are equal. Instead, there is one Divine Nature only, which is fully possessed by each of the Three Persons. This makes the Three Persons co-equal as God. Each Person is fully God.
But there are not four things in God: Person, Person, Person, Nature. Rather, the three Persons are the one Nature, and the one Nature is the three Persons. Despite the thorough oneness of God, somehow perfect infinite eternal divine unity becomes three.
Now the Divine Nature is perfect infinite eternal goodness. All that is good is One in God, and this goodness is infinite and eternal. And there is only one God. And there can only be one God. Then all the things that we can say about the Divine Nature are One. God exists, God loves, God is all knowing, God is all powerful, etc. All these things are One in the Divine Nature. In God, existence is love is knowledge is mercy is justice is power, etc. And yet the One Nature is the Three Persons. Somehow this absolute oneness of God is inherently three Persons.
But let’s continue discussing the one Nature. What if we propose a second God, who is also perfect infinite eternal goodness. How would we distinguish between the two? What would be the difference between the one and the other? Having one God who is all goodness to all perfection infinitely and for eternity, nothing is left for the other God. All that could possibly be of the Divine Nature is in the One God, and nothing is left for a second God. So there can only be one God. A second God would be indistinguishable from the first, and would in fact be the first. A second God is impossible.
So then, what distinguishes the Three Persons of the Trinity, so that there can be three who are not the same? If a second Divine Nature is impossible, how can there be a second and third Person, and not a fourth or fifth Person?
The Father does not proceed. The Son proceeds only from the Father. The Holy Spirit proceeds from the Father and from the Son. The Spirit proceeds from Father and Son as from one principle; it is not a double procession. And then we can also say that no Person proceeds from the Spirit; One Person proceeds from the Son; Two Persons proceed from the Father.
What makes each Person in the Trinity unique and therefore able to exist as a distinct Person in the one eternal God is their relationships with one another.
If the Holy Spirit were to proceed only from the Father, then the Spirit would be just like the Son, an Eternal Word as if spoken by the Father. But having two Eternal Words, each of whom is infinite perfection would mean — just as in the case of the hypothetical second God — that nothing would make the one Word (Son) different from the other Word (Spirit). The Son proceeds only from the Father. So if the Spirit proceeds only from the Father, then the Spirit and the Son would be identical, which is not possible. Two identical Persons cannot exist, since each is infinite perfection. Without a difference based on procession, there cannot be two who are the same.
But since the Holy Spirit proceeds from the Father and Son, this distinguishes the Spirit from the Son. Then each of the Three Persons differs from one another in their relationships to one another. And this is what allows there to be three Persons in one God.
The filioque clause was added to the Creed so as to specify that the Holy Spirit proceeds from the Father and the Son (“Filioque” means “and the Son”). This is absolutely necessary to the identity of the Spirit and to His unique Personhood. It is what makes the Spirit not another Son, but the Spirit of God. The very identity of the Holy Spirit depends on His procession from Father and Son. Those who reject the filioque clause must repent and accept this teaching. It is a crucial dogma for understanding, as much as we finite creatures are able to understand, about the mystery of the Trinity.
Can there be a fourth or fifth Person? No. This is divinely revealed. We can use our faculty of reason to consider this question, but without Divine Revelation, we could not be sure that God is any particular number of Persons. It is revealed Truth. We can reason to the fact that God exists, and that He is One, but we cannot reason to the fact that the One God is Three Persons.
If there were a fourth person, how would He differ from the other Persons by procession, which is the basis for the Personhood of each in the Trinity? Could a Person proceed from three persons, rather than from none (the Father), one (the Son), or two (the Spirit)? No. For then there would be no reason why there would not be a fifth person, proceeding from four, and a sixth person, proceeding from five, etc. And an infinite number of Persons in God would be chaotic and absurd, having no end to their number. This would be like a division and shattering of the Divine Nature, which is not possible.
God has always existed, so there can be a Person, the Father, who does not proceed. The Father knows himself with infinite perfection, and so there has always existed the Word of God, the Son. Then the Father and Son love one another, and so there has always existed the Spirit of God. But the knowledge of one another or the love for one another of the Three Persons does not result in another procession, as a procession based on (or analogous to) knowledge and love already exist. Thus, there cannot be a fourth or fifth or more Persons in God.
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