Chapter C7, “Ecclesiastical Terminology and the Christian Knowledge of God”
The Christian faith is, in the final analysis, highly practical. God has made peace with man. How do we receive that and live it out? Pieper asks about the purpose of very specific theological and churchly terms which some have said lead to a jargonization of the Church. While these specific terms are not strictly necessary they may well be valuable due to their clarity and specificity. He then discusses seven which he considers most important.
1) Trinity - the specific term for the one God who exists in the persons of Father, Son, and Holy Spirit.
2) Person - the specific individual within the Godhed, as opposed to a mode of operation, attribute, etc.
3) Essence - There is one divine essence. All three persons of the Trinity share the same essence. Pieper doesn’t mention it, but this is really simply a translation of the Greek verb of being. It signifies the state of being God.
4) Consubstantial - being of one essence. Again, this describes how the persons of the Trinity share their essence.
5) Filioque - used to describe the Holy Spirit proceeding from the Son as well as from the Father. The term was added to the Nicene Creed by the Western Church in 589. The idea is clearly present in Scripture.
6) circumincessio - immanence - the three persons of the Trinity, having one essence, interact together in complete unity because of the shared essence.
7) opera divina ad intra - divine works within the Trinity / opera divina ad extra - divine works relating to the world. We observe that the divine works relating to the world can be addressed as belonging to the realm of one person of the Trinity bu those ad intra are not distinguished as to the source.