[1] These are the terms employed by St. Thomas Aquinas in the Summa Theologica, Ia q. 36 a. 3 1um and 2um.
[2] It is Tertullian who lays the foundations for Trinitarian theology in the Latin tradition, on the basis of the substantial communication of the Father to the Son and through the Son to the Holy Spirit: "Christ says of the Spirit 'He will take from what is mine' (Jn 16:14), as he does from the Father. In this way, the connection of the Father to the Son and of the Son to the Paraclete makes the three cohere one from the other. They who are one sole reality (unum) not one alone (unus) by reason of the unity of substance and not of numerical singularity" (Adv. Praxean, XXV, 1-2). This communication of the divine consubstantiality in the Trinitarian order he expresses with the verb procedere (ibid., II, 6). We find this same theology in St. Hilary of Poitiers, who says to the Father: "May I receive your Spirit who takes his being from you through your only Son" (De Trinitate XII, PL 10, 471). He remarks: "If anyone thinks there is a difference between receiving from the Son (Jn 16:15) and proceeding (procedere) from the Father (Jn 15:26), it is certain that it is one and the same thing to receive from the Son and to receive from the Father (De Trinitate, VIII, 20, PL 10, 251 A). It is in this sense of communication of divinity through procession that St. Ambrose of Milan is the first to formulate the Filioque: "The Holy Spirit when he proceeds (procedit) from the Father and the Son, does not separate himself from the Father and does not separate himself from the Son" (De Spiritu Sancto, 1, 11, 120, PL 16, 733 A = 762 D). St. Augustine, however, takes the precaution of safeguarding the Father's monarchy within the consubstantial communion of the Trinity: "The Holy Spirit proceeds from the Father as Principle (principaliter) and, through the latter's timeless gift to the Son, from the Father and the Son in communion (communiter)" (De Trinitate XV , 25, 47, PL 42, 1095). St. Leo, Sermon LXXV, 3, PL 54, 402; Sermon LXXVI, 2, ibid. 404).
[3] Tertullian uses the verb procedere in a sense common to the Word and the Spirit insofar as they receive divinity from the Father: "The Word was not uttered out of something empty and vain, and he does not lack substance, he who proceeded (processit) from such a (divine) substance and has made so many (created) substances. (Adv. Praxean, VII, 6). St. Augustine, following St. Ambrose, takes up this more common conception of procession: "All that proceeds is not born, although what is born proceeds" (Contra Maximinum, II, 14, 1, PL 42, 770). Much later St. Thomas Aquinas remarks that "the divine nature is communicated in every processing that is not ad extra (Summa Theologica Ia, q.27, a.3, 2um). For him, as for all this Latin theology which used the term "procession" for the Son as well as for the Spirit, "generation is a procession which puts the divine person in possession of the divine nature" (ibid., Ia. q.43, a 2, c), for "from all eternity the Son proceeds in order to be God" (ibid.). In the same way, he affirms that "through his procession, the Holy Spirit receives the nature of the Father, as does the Son (ibid., Ia, q.35, a.2, c). "Of words referring to any kind of origin, the most general is procession. We use it to indicate any origin whatever; we say, for instance, that the line proceeds from the point; that the ray proceeds from the sun, the river from its source, and likewise in all kinds of other cases. Since we admit one or another of these words that evoke origin, we can, therefore, conclude hat the Holy Spirit proceeds from the son (ibid., Ia, q.36, a.2, c).
[4] St. Cyril bears witness here to a Trinitarian doctrine common to the whole school of Alexandria since St Athanasius, who had written "Just as the Son says: 'All that the Father has is mine' (Jn 16:15), so shall we find that, through the Son, it is all also in the Spirit" (Letters to Serapion, III, 1, 33, PG 26, 625 B). St. Epiphanius of Salamis (Ancoratus, VIII, PG 43, 29 C) and Didymus the Blind (Treatise on the Holy Spirit, CLIII, PG 34, 1064 A) link the Father and the Son by the same preposition ek in the communication to the Holy Spirit of the consubstantial divinity.
[5] "The two relationships of the Son to the Father and of the Holy Spirit to the Father oblige us to place two relationships in the Father, one referring to the Son and the other to the Holy Spirit" (St. Thomas Aquinas, Summa Theologica, Ia, q.32, a.2, c).
[6] Cf. Catechism of the Catholic Church, no.248.
[7] St. Gregory of Nazianzus says that "the Spirit is a middle term (meson) between the Unbegotten and the Begotten" (Discourse 31, 8, Sources Chrétiennes, no.250, p.290). Cf. also, in a Thomistic perspective, G Leblond, "Point of view on the procession of the Holy Spirit," in Revue Thomiste, LXXXVI, t.78, 1978, pp.293-302.
[8] St. Cyril of Alexandria says that "the Holy Spirit flows from the Father into the Son (en to Uiou)," (Thesaurus, XXXIV, PG 75, 577A).
[9] St. Gregory of Nyssa writes: "The Holy Spirit is said to be of the Father and it is attested that he is of the Son. St. Paul says: 'Anyone who does not have the Spirit of Christ does not belong to him' (Rom 8:9). So the Spirit who is of God (the Father) is also the Spirit of Christ. However, the Son who is of God (the Father) is not said to be of the Spirit: the consecutive order of the relationship cannot be reversed" (Fragment In orationem dominicam, quoted by St. John Damascene, PG 46. 1109 BC). And St. Maximus affirms in the same way the Trinitarian order when he writes: "Just as the Thought (the Father) is principle of the Word, so is he also of the Spirit through the Word. And, just as one cannot say that the Word is of the voice (of the Breath), so one cannot say that the Word is of the Spirit" (Quaestiones et dubia, PG 90, 813 B).
[10] St. Thomas Aquinas, who knew the De Fide Orthodoxa, sees no opposition between the Filioque and this expression of St. John Damascene: "To say that the Holy Spirit reposes or dwells in the Son does not exclude his proceeding from the Son; for we say also that the Son dwells in the Father, although he proceeds from the Father (Summa Theologica, Ia, q.36, a.2, 4um).
[11] St. Thomas Aquinas, following St. Augustine, writes: "If we say of the Holy Spirit that he dwells in the Son, it is in the way that the love of one who loves reposes in the loved one" (Summa Theologica Ia, q.36, a.2, 4um). This doctrine of the Holy Spirit as love has been harmoniously assumed by St. Gregory Palamas into the Greek theology of the ekporeusis from the Father alone: "The Spirit of the most high Word is like an ineffable love of the Father for this Word ineffably generated. A love which this same Word and beloved Son of the Father entertains (chretai) towards the Father: but insofar as he has the Spirit coming with him (sunproelthonta) from the Father and reposing connaturally in him" (Capita physica XXXVI, PG 150, 1144, D-1145 A).
[12] Cf. John Paul II, Encyclical Dominum et Vivificantem nn.18-24, AAS LXXVIII, 1986, 826-831. Cf. also Catechism of the Catholic Church, nos. 438, 689 690, 695, 727.