Theses theologicae de sacrosancta trinitate contra antitrinitarios

XLV.
For the fact that persons are said to exist in the divine essence does not mean they are in it as an accident in a subject, but insofar as all and each communicate in the one and indivisible divine essence.

XLVI.
Thus, what we read in John 14: The Father is in me, and I in the Father: is not to be accepted as if the Son is in the Father as a quality or accident in a subject, as Servetus blasphemously taught.

XLVII.
For if this were so, that reciprocation would not be valid: The Father in me, and I in the Father: since an accident is in a subject, but the subject is not in the accident.

XLVIII.
Nor is one person in another as a τόδε τι particular/this thing, but the Father is said to be in the Son, and the Son in the Father, because of the περιχώρησιν interpenetration/circumincession of the one and indivisible essence, just as Christ explicitly explains this phrase in John 10 and says: I and the Father are one.

XLIX.
But the cause of the ὑποστάσεως hypostasis/person is not said to be the Son in the Father, but with the Father, John 17, 1 John 1.

L.
Although other created persons also have this in common with divine persons—that they are neither predicated of another, nor are they in another as an accident in a subject—yet it differs in that created persons, because of the concretion and composition of matter, are also essentially distinct, and separated by the number of essences, having only a logical μέθεξιν participation and community. But in the Trinity, which is the principle of principles and is devoid of all matter and creation, a person signifies an individual which distinguishes itself from other persons of the same Trinity by the τρόπῳ ὑπάρξεως mode of existence, but communicates with them by οὐσίᾳ essence.

LI.
Furthermore, care must be diligently taken that we do not abstract the persons from the divine essence, by which reasoning a fourth God would be feigned, whom the Antitrinitarians absurdly call τετράδα a quaternity, who would neither be the Father alone, nor the Son alone, nor the Holy Spirit alone.

LII.
For those three persons are nothing other than God Himself simply: and conversely, the divine essence is nothing other than the Father, the Son, and the Holy Spirit joined together. Which Nazianzen elegantly discusses in his sermon on Baptism in these words: ὁ φθάνω τὸ ἓν νοῆσαι, καὶ τοῖς Before I begin to conceive the One, I am illuminated by the Three...