More on the filioque. While it’s important, consider it more of an intellectual exercise in our day. We have much greater things to argue over.
1 There are some, pertinacious in their willful resistance to the truth, who make some points to the contrary which are hardly worth an answer. They say that our Lord, speaking of the procession of the Holy Spirit, says that He proceeds from the Father, without mentioning the Son. So one reads in John (15:26): “When the Paraclete cometh, whom I will send you from the Father, the Spirit of truth, who proceeds from the Father.” Hence, since nothing must be held about God which is not given in Scripture, it must not be said that the Holy Spirit proceeds from the Son.
2 But this is entirely frivolous. For, by reason of unity of essence, what is said in the Scriptures about one Person ought to be understood of another, unless it is repugnant to His propriety as a Person, and this even if some exclusive phrase is added. For, although it says in Matthew (12:27): “No one knows the Son, but the Father,” neither the Son nor the Holy Spirit is, for all that, excluded from knowledge of the Son. Hence, even if it is said in the Gospel that the Holy Spirit does not proceed from any but the Father, this would not exclude His proceeding from the Son. For this is not repugnant to the propriety of the Son, as was shown.
Neither is there cause to marvel if our Lord said that the Holy Spirit proceeds from the Father, saying nothing about Himself, His custom is to refer everything to His Father from whom He has whatever He has. Thus, He says in John (7:16): “My doctrine is not Mine, but His that sent Me.” Many things of this kind are discovered in the words of our Lord which establish in the Father the authority of the principle. And, for all that, in the passage just mentioned our Lord was not altogether silent about His being the principle of the Holy Spirit. He called Him “the Spirit of Truth,” and He had previously called Himself “the Truth” (John 24:6).
3 They further object that in certain councils one finds it prohibited under penalty of anathema to add anything to the Creed ordered by the council. In this, they say, there is no mention of the procession of the Holy Spirit from the Son. And so they hold the Latins guilty of anathema because they have added this to the Creed.
4 But such arguments are inefficacious. For the declaration of the Synod of Chalcedon says that the Fathers gathered at Constantinople corroborated the doctrine of the Synod of Nicea. This they did, “not as though to imply that the doctrine was something less, but to declare by Scriptural testimonies the understanding of the Holy Spirit Of their predecessors against those who attempted to reject that understanding.”
One must say, similarly, that the procession of the Holy Spirit from the Son is implicitly contained in the Creed of Constantinople, for the latter says that “He proceeds from the Father,” and what is understood of the Father must be understood of the Son, as was said. And the authority of the Roman Pontiff sufficed for this addition; by this authority, too, all the ancient councils were confirmed.
5 They maintain, also, that the Holy Spirit, since He is simple, cannot be from two; and that the Holy Spirit, if He proceeds perfectly from the Father, does not proceed from the Son; and other arguments of this sort. These are easy to solve, even if one is but little skilled in theological matters. For the Father and the Son are a single principle of the Holy Spirit by reason of the unity of divine power, and by one production they produce the Holy Spirit; thus, also, the three Persons are one principle of creatures and by one action they produce creatures.