The doctrine of perichoresis (“perry-CORE-ee-sis”) or “circumincession” in Latin, says that each Hypostasis in the Holy Trinity eternally lives and dwells within the Others. Father, Son, and Holy Spirit each, so to speak, occupy the same space. They co-inhere by virtue of each possessing the same Substance. St. John of Damascus (Exact Exposition of the Orthodox Faith 1.14) explains:
The subsistences [i.e., the three Persons] dwell and are established firmly in one another. For they are inseparable and cannot part from one another, but keep to their separate courses within one another, without coalescing or mingling, but cleaving to each other. For the Son is in the Father and the Spirit: and the Spirit in the Father and the Son: and the Father in the Son and the Spirit, but there is no coalescence or commingling or confusion. And there is one and the same motion: for there is one impulse and one motion of the three subsistences, which is not to be observed in any created nature.The main basis for the doctrine of perichoresis is Jesus’ saying, "I am in my Father, and my Father is in me" (John 14:10) as well as His prayer in John 17, which is, in part:
I do not pray for these alone, but also for those who will believe in Me through their word; that they all may be one, as You, Father, are in Me, and I in You; that they also may be one in Us, that the world may believe that You sent Me. And the glory which You gave Me I have given them, that they may be one just as We are one: I in them, and You in Me; that they may be made perfect in one, and that the world may know that You have sent Me, and have loved them as You have loved Me.
Father, I desire that they also whom You gave Me may be with Me where I am, that they may behold My glory which You have given Me; for You loved Me before the foundation of the world. O righteous Father! The world has not known You, but I have known You; and these have known that You sent Me. And I have declared to them Your name, and will declare it, that the love with which You loved Me may be in them, and I in them. (John 17:20-26)
Perichoresis means Father, the Son, and the Holy Spirit eternally pour Themselves out, Each to the other Two, in total, self-donating Love. It means that God always loved, without need of creating anything to be loved by Him. Each Hypostasis is a fountain of overflowing love; the Trinity is a constant movement and communion of Love.
As a Communion of Love, the Holy Trinity is the very archetype of personhood. To be a human person, to fulfill ones humanity, is to live in a communion of love with every other and with God, such that we all live within one another (Romans 12:5) and God.
Salvation means for human beings to be drawn into God’s own perichoresis, to participate in His dance of love.