(Taken from Notes Made at the Conferences of Dom Prosper Guéranger)
Qui ex Patre Filioque procedit: this same Holy Ghost proceedeth from the Father and the Son. How could one suppose that the Father and the Son are not united? There needs must be a link uniting One to the Other. The Father and the Son are not merely in juxtaposition, but a Link unites Them, embraces Them, and this Link proceeds from Both of Them, forming but One with Them; and this Mutual Love is no other than the Holy Ghost.
At the Council of Nicaea, in drawing up the Symbol, the main attention of the Fathers was directed to what treated of Jesus Christ; at the Council of Constantinople, they resolved upon completing the Nicene Creed, by adding what regards the Holy Ghost, save the words Filioque; as they expressed it, the words simply stood Qui ex Patre procedit. The Fathers of this Council saw no necessity of saying more on the subject of the Procession, because the words of Our Lord, in the Gospel, leave no doubt on the matter. “I will send you the Spirit of Truth who proceedeth from the Father:” Ego mittam vobis a Patre Spiritum veritatis qui a Patre procedit. (S. John, xv. 26): He is therefore, likewise the Principle of the Holy Ghost, as He sends Him. The Father sends the Son, and it is evident that the Son is begotten by the Father; Our Lord here saying: “I will send you the Spirit,” proves that He is Himself the Source of the Holy Ghost, as is the Father. And if our Lord adds these words: Qui a Patre procedit, He in no way means to say that the Holy Ghost proceeds from the Father only; it is merely in order to give further expression to His own words, and to emphasize that not He alone sends this Divine Spirit, but that the Father, conjointly with the Son, sends Him.
The Greeks refused to admit this Truth, and so raised disputations on this passage, in order to overturn the Dogma of the Trinity. But we believe that the Trinity is linked in Its Three Persons, in this ineffable manner, namely, that the First Person begets the Second; the First and the Second are united to one another by the Third. If belief be refused in this Bond produced by the Father and the Son, and linking Them together, the Holy Ghost would be utterly isolated from the Son, the Trinity would be destroyed.
It was in Spain that the addition of the Filioque was first of all introduced into the Creed, in order to express with greater precision what the Fathers of Constantinople had declared; this change was begun in the eighth century; but the Roman Church did not adopt it till the eleventh. She knew that such a measure would provoke difficulties; but seeing the necessity, she decided upon it, and since then, this addition to the Symbol has become obligatory on the whole [Western] Church.