Barth, along with much of the Western tradition, defends the filioque on the basis of coherence of the economic and ontological Trinity. If God is not as He appears, we have no revelation of God.
John Zizioulas responds by opening up a rather surprising gap between economy and ontology. He acknowledges that “we cannot say that the immanent Trinity is one thing and the economic Trinity is entirely another.” Yet, the focus of his remarks is on the difference between them:
“for the immanent Trinity we cannot say anything definitive about the attributes of the persons . . . . In the economic Trinity we can say specific things about the attributes of the persons, because these persons have taken on these particular tasks and characteristics freely for us. Though the Son is the revelation of the Father . . . this does not necessarily mean that the Son has this function and attribute in the eternal Trinity. If the Spirit signifies love and creates communion for us, this does not mean this is due to a specific attribute of the Spirit in the eternal Trinity. The persons take on these attributes freely for our sake, so they relate to the economy, the way God is for us.” In the immanent Trinity, “we cannot say that one person is love and the other is knowledge. They are not attributes but operations of persons acting together in freedom, and all the operations of the eternal Trinity are one.”
This seems to be in some tension with Zizioulas’ adherence to Cappadocian Trinitarianism. As I understand the Cappadocian position, the Persons are distinct eternally because of their personal particularities and relations of origin. I suppose that Zizioulas would distinguish relations of origin from internal actions and attributes; it’s one thing to say that the Son bears the personal particularity of “filiation,” another to say that He is the Father’s “revelation” within the immanent Trinity.
More importantly, Scripture doesn’t Zizioulas’ apophatic strictures on speech about the Person’s “attributes” within the immanent Trinity. True, the Bible never calls the Son the eternal “revelation” of the Father, but it does call Him “Word” and “Image” and “Radiance” and “exact Character of his glory.” It is not accidental, or merely a “free” choice for the Son to be the One Incarnate; as Son, He is the Person who is incarnate-able, just as the Spirit is eternally the indwelling-able Person, as as the Father as the begetter of the Son is eternally the Person who is Sender-able.