Introduction
First, we must establish what the Filioque means before we argue for the acceptance of the phrase. The doctrine comes from the Latin version of the Nicene creed, where the term “Filioque” (and the Son) appears after “Proceeds from the Father” in the original version.
There are multiple ways in which this phrase can be interpreted. In its most extreme form, it is interpreted to mean that there are two separate principles of the Holy Spirit, the Father, a single principle of spiration, and the Son, another principle of spiration.
Latin theology (mostly) has rejected this interpretation in favor of a more “moderate” reading of the phrase. That is, the Father and the Son form, together, one principle of the Holy Spirit. The Father is the “immediate cause” and the Son the “mediate cause” of the Holy Spirit. This is helpfully described as “from the Father through the Son,” glossing “que” (and) to mean “per” (through).
It is this interpretation of the Filioque that will be defended in this article. It is important to note that great strides have been made been the Latins and Greeks on this issue of the Filioque. This language of “one shared principle” and “through the Son” has been received by many in the east.
The Argument
The central reason why the theology of the Filioque must be retained is that otherwise, we would not be able to distinguish the hypostasis (person) of the Son and the hypostasis of the Spirit. In brief, the persons of the Trinity are only distinguished from their processions; the Son proceeds from the Father alone (as all admit); therefore, the Spirit could not proceed from the Father alone, or else, there would be no way of distinguishing them.
The first premise in this argument is that the persons of the Trinity can only be distinguished by their subsisting relations of origin and procession. It is admitted by all that they must be able to be distinguished. This distinction cannot be of substance, or else, the Trinity is split into three substances, and we would have three gods. There would be no “consubstantial.” Therefore, as Boethius says, “quod sola relatio multiplicat Trinitatem divinarum personarum” (Relation alone multiplies the Trinity of the divine persons).
Further, these relations must also be opposite relations (i.e., from x to y) in order to truly distinguish. This is vital to the thesis given above. For one may argue, “the Son and Spirit may be distinguished from the fact that they are different processions from the Father.” This does not suffice because they must be opposite relations. For, in this scheme, they both have opposite relations to the Father (and thus can be distinguished from Him). However, they do not have opposite relations to each other.
This appears from the case of the Father. For, He has two relations, first, from the fact that He generates the Son, and second, from the fact that He spirates the Spirit. Why, then, do we not distinguish these into two Fathers? We do not distinguish because these relations are not opposite relations.
Therefore, if we are to only distinguish the Son and Spirit from the fact that they both proceed from the Father, we would have no principle of distinction because these are not opposite relations. We would have as much basis of distinguishing the Son and Spirit as we would Father 1 and Father 2.
Therefore, we must not only posit an opposite relation of the Son to the Father and the Spirit to the Father but also of the Son to the Spirit.
Lastly, the only opposite relations that can distinguish are relations of origin. For, a relation can either be of quantity (half/double) or of act (doer/done to). We know it cannot be the former, for there is no quantity in God that can be split up (and this would lead to Tritheism).
Therefore, it must be of act. With act, it can either be external or internal. It cannot be external, for that would break the unity of God; therefore, it must be internal actions, i.e., procession and origin.
Therefore, it must be an internal act (procession and origin) wherein the two persons stand in opposition to one another. This can either be achieved, a., the Son from the Father and the Spirit (which is discounted easily), or b. the Spirit from the Father and the Son, which is established by elimination and necessary consequence.
Conclusion
In sum, the Filioque is the doctrine that the Spirit is from the Father through the Son. This is established through necessary conditions of Trinitarian theology. First, from the fact that the Spirit and Son must be distinguished. Second, from the fact that this distinction is only achieved by an opposite relation of active and passive spiration between the two.