Theologicae regulae

This preposition, "in," in divine matters, sometimes denotes identity of essence and plurality of person; sometimes a kind of diversity; sometimes eternal predication; sometimes identity; sometimes efficient cause; sometimes, as it were, formal cause.

For example, when it is said: The Son is in the Father, thus is denoted plurality of person along with identity of essence. For if, when it is said: Atalanta is reflected in Parthenopaeus, an express likeness of nature is denoted; it is not absurd if, along with the plurality of person, identity of nature is denoted, when it is said: The Son is in the Father. As it were diversity, when it is said: Deity is in the Father, I do not say diversity, but as it were diversity: for there is no diversity between deity and the Father, since deity is the Father, and the Father is deity; but a certain transition is denoted, as it were, when the preposition is there placed by apposition. Eternal predication, when it is said: "What was made, in him was life,"¹ as it were, in him lived, by an eternal disposition, whatever was to come. Efficient cause, when it is said: In the Son all things were made, that is, through the Son. As it were formal cause, when it is said: "Who, since he was in the form of God"². Identity, according to which it is said that God, before the constitution of the world, was in himself, or with himself.