2. The demonstration of the filioque: “double procession.” The traditionally Western trinitarian concept of the double procession of the Holy Spirit was consistently upheld by the Reformers and argued with some vigor against the Greek Orthodox view. The Reformed exegetes, moreover, understood the issue to be one of exegesis, not merely an issue of the form of the Niceno-Constantinopolitan Creed, and found the biblical text to be entirely of one accord in favor of double procession. Vermigli writes, with reference to John 15:26,

Seeing the Son saith, that he will send the Spirit, and (as we said before) affirmeth him to receive of his; no man doubteth, but that he proceedeth from the Son. And now he expressly addeth; Who proceedeth from the Father.

Calvin took the point with equal seriousness, noting in his commentary on the same text,

When he says that he will send him from the Father, and, again, that he proceedeth from the Father, he does so in order to increase the weight of his authority; for the testimony of the Spirit would not be sufficient against attacks so powerful, and against efforts so numerous and fierce, if we were not convinced that he proceedeth from God. So then it is Christ who sends the Spirit, but it is from the heavenly glory, that we may know that it is not a gift of men, but a sure pledge of Divine grace. Hence it appears how idle was the subtlety of the Greeks, when they argued, on the ground of these words, that the Spirit does not proceed from the Son; for here Christ, according to his custom, mentions the Father in order to raise our eyes to the contemplation of his Divinity.

As in Vermigli’s comment, Calvin’s analysis of the text assumes the sending of the Spirit by Christ and therefore the procession of the Spirit from the Son and views the further statement of the Gospel that the Spirit proceeds from the Father not restrictively but as an expansion of the meaning to include the Father.

Calvin rather emphatically takes the words “he proceeds from the Father” as an indication of the authority of the Spirit, not of the sole origin of his eternal procession: Christ here sends the Spirit, but manifests the Spirit as a “sure pledge of divine grace.” It is, he concludes, an “idle subtlety of the Greeks” to claim this text as warrant for their denial of double procession. Calvin points out in his comment on Romans 8:9,

But let readers observe here, that the Spirit is, without any distinction, called sometimes the Spirit of God the Father, and sometimes the Spirit of Christ; and thus called, not only because his whole fulness was poured on Christ as our Mediator and head, so that from him a portion might descend on each of us, but also because he is equally the Spirit of the Father and of the Son, who have one essence, and the same eternal divinity.

The orthodox follow the Reformers in upholding the Western doctrine of the filioque. The orthodox Reformed writers not only argue the Augustinian doctrine of double procession they insist on it as a biblical point held over against the teachings of the Greek Orthodox:

The property of the Son in respect of the Holy Ghost is to send him out, John 15:26. Hence arose the Schism between the Western and the Eastern Churches, they affirming the procession from the Father and the Son, these from the Father alone (Edward Leigh, Treatise; Downame, Summe)

Among the Reformed orthodox theologians, Pictet notes the clear distinction of persons in John 15:26:

Here the Comforter, or Spirit, is plainly distinct from the Father and the Son. Again, they are so distinguished, that some things are said of the Father which cannot be said of the Son, and some things of the Son which are no where said of the Spirit. The Father is said to have begotten the Son … the Spirit is said to proceed from the Father, and to be sent by the Son; but nowhere is the Father said to proceed from nor the Son to be sent by the Spirit. Yet are these persons distinct in such a manner, that they are not three Gods but one God; for the scripture everywhere proves and reason confirms, the unity of the Godhead.

Similar statements are found among the Reformed exegetes of the era. Poole notes that the text has been read variously: some exegetes understand the Spirit’s procession from the Father merely as his coming forth or being poured out at Pentecost, whereas others—“the generality of the best interpreters”—understand the text as a reference to “the Holy Spirit’s eternal proceeding.” Owen, by way of contrast, argues the primary meaning of the text to be that the Spirit “goeth forth or proceedeth” in order to “put into execution” the salvific counsel of God in the application of grace and views the immanent procession of the Spirit as a secondary meaning, a conclusion to be drawn from the text.

As Pictet notes, the Reformed orthodox uniformly follow the Western doctrine:

That the Spirit proceeds from the Son, is proved by those passages in which he is represented as being sent no less by the Son than by the Father; nor is he any less the Spirit of the Son than of the Father: Rom. 8:9, “any one who does not have the Spirit of Christ …; Gal. 4:6, “God has sent the Spirit of his Son into our hearts”; John 16:7, “If I do not go away, Comforter will not come to you, but if I go, I will send him to you.”

Nor is this a minor point in theology that can be dismissed:

To deny the procession of the Holy Ghost from the Son, is a grievous error of Divinity, and would have grated the foundation, if the Greek Church had so denied the procession of the Holy Ghost from the Son, as that they had made an inequality between the Persons. But since their form of speech is, that the Holy Ghost proceedeth from the Father by the Son, and is the Spirit of the Son, without making any difference in the consubstantiality of the Persons it is a true though erroneous Church in this particular; divers learned men think that à Filio & per Filium in the sense of the Greek Church, was but a question in modo loquendi, in manner of speech, and not fundamental.

The problem of the filioque was, therefore, not something that the Reformed orthodox could ignore: they refused to go so far as to claim that the Greek church was a false church, but they still insisted that it ensconced an error in its doctrinal explanations of the creed.

From the Reformed perspective, moreover, the Greek critique of the filioque, that it implied two ultimate principia or archai in the Godhead, did not hold—for there could only be two archai if the Father and the Son separately and equally were the sources of the Spirit’s procession. The orthodox conception of the filioque, however, insisted on the unity of the act of the Father and the Son, so that the Holy Spirit proceeds from Father and Son by “one and the same breathing” and does so from both equally, the Father and the Son acting in communion with one another. Thus, the Holy Ghost, the third person, proceeds from the Father and the Son: “and albeit the Father and the Son are distinct persons, yet they are both but one beginning of the holy Ghost.” At the same time, following the Western pattern, the Reformed orthodox insisted on the begetting of the Son as placing the Son second in order, thus maintaining the Father as ultimate source of the personal distinctions and the Father and the Son together as the source of the Spirit.

—Richard Muller, Post Reformation Reformed Dogmatics 4.373–76 (via Logos edition)