The internal lines identify the nature, substance, or essence of each person:
- The Father is God.
- The Son is God.
- The Holy Spirit is God.
As Basil of Caesarea writes in the 370s (Letter 236.6):
The distinction between ousia and hupostasis is the same as that between the general and the particular; as, for instance, between the animal and the particular man.
Wherefore, in the case of the Godhead, we confess one essence or substance so as not to give a variant definition of existence, but we confess a particular hypostasis, in order that our conception of Father, Son and Holy Spirit may be without confusion and clear. If we have no distinct perception of the separate characteristics, namely, fatherhood, sonship, and sanctification, but form our conception of God from the general idea of existence, we cannot possibly give a sound account of our faith.
We must, therefore, confess the faith by adding the particular to the common. The Godhead is common; the fatherhood particular. We must therefore combine the two and say, I believe in God the Father.
The like course must be pursued in the confession of the Son; we must combine the particular with the common and say I believe in God the Son, so in the case of the Holy Ghost we must make our utterance conform to the appellation and say in God the Holy Ghost.
The lines of the triangle represent two sets of propositions. First, they remind us that while each of the persons in the Godhead is God (fully divine), the persons are at the same time distinct. In other words:
- The Father is not the Son.
- The Son is not the Father.
- The Father is not the Holy Spirit.
- The Holy Spirit is not the Father.
- The Son is not the Holy Spirit.
- The Holy Spirit is not the Son.
After all, the Father is never “sent” in Scripture. Nor is he incarnated or poured out at Pentecost. The Spirit does not die on the cross for our sins. The Father begets the Son, not vice-versa. The Spirit proceeds from the Father and the Son.
Another aspect indicated by the lines on the triangle is that of mutual indwelling (or perichoresis). The three persons indwell each other in the one being of God. So:
- The Father is in the Son.
- The Son is in the Father.
- The Father is in the Holy Spirit.
- The Holy Spirit is in the Father.
- The Son is in the Holy Spirit.
- The Holy Spirit is in the Son.
Finally, each of the three persons in the one being of God glorify one another. As Gregory of Nyssa writes, there is a “revolving circle” of glory:
The Son is glorified by the Spirit; the Father is glorified by the Son; again the Son has His glory from the Father; and the Only-begotten thus becomes the glory of the Spirit. . . . In like manner, again, Faith completes the circle, and glorifies the Son by means of the Spirit, and the Father by means of the Son. (Gregory of Nyssa, On the Holy Spirit, in NPNF, Second Series, 5:324).