Previously, we have discussed about Unity in Trinity and Trinity in Unity. Now, the dialectic: The triunity (die Dreieinigkeit)! “[T]he one God may be known only in the Three and the Three only as the One God, so none of the Three may be known without the other Two but each of the Three only with the other Two.” (I/1, 370) We could not speak about the Father without eventually speaking about the Son and the Spirit. We could not speak about the Son without eventually speaking about the Father and the Spirit. We could not speak about the Spirit without eventually speaking about the Father and the Son. They are inseparable: the Father is in the Son and the Spirit, the Son is in the Father and the Spirit, and the Spirit is in the Father and the Son. This kind of dialectic between Unity in Trinity and Trinity in Unity is expressed with the doctrine of perichoresis, as “the divine modes of being mutually condition and permeate one another so completely that one is always in the other two and the other two in the one.” (370) This kind of Unity in Trinity and Trinity in Unity is, indeed, unfathomable and ineffable.
How does one describe the unity of the Trinity and the distinction of modes of being in God so that it would truly speak of God in himself, then? Well, we would speak of him as he is revealed to us in his acts, since his acts and his essence are one. But the works of God for us as attested in the Scripture do not exhaust the essence of God for himself, as God is free in giving Himself for us. Yet, it does not hinder us to speak about God for himself from God for us. And, this analogy between God for us and God for himself is called the doctrine of appropriation, i.e., to talk appropriately of God. For example, we speak of the Father as the Creator, the Son as the Reconciler, and the Spirit as the Redeemer. Or, that, according to the Pauline benediction: grace to the Son, love to the Father, and fellowship to the Spirit (2 Cor 13.13). Or, that we pray to the Father through the Son in the Spirit. Of course, it does not mean that only the Father who is the Creator, as the same could be said as well for the Son and the Spirit. Nor that it means that we could not say love of the Son or love of the Spirit. Nor that it means that we could not pray to the Son or to the Spirit. Nevertheless, we must speak of the Father as the Creator, the Son as the Reconciler, and the Spirit as the Redeemer as the distinction of modes of being in God is real as well. And we do pray to the Father through the Son in the Spirit.
What are the rules of appropriation which is proper to the essence of God (I/1, 374)? (1) The appropriation must not be arbitrary as not every triad could be adapted to denote truthfully the mystery of the Trinity, (2) it must not be exclusive, as the appropriation of a particular mode of being of God must not be made an exclusive property of this mode of being, and (3) it must not be invented freely, as it owned its origin and source in the witness to revelation attested in the Scripture (I think the third criterion could be combined with the first criterion). The archetype of the appropriation, of course, is the triad of the Father, the Son, and the Holy Spirit.
Nevertheless, to repeat the point previously mentioned, eventually these proper talks of God will be found improper, as it could not exhaust the essence of God in himself. Theology proper is improper.