Jesus said, “I and the Father are one.”  He then went on to say, “…the Father is in me, and I in the Father.”  Theologians have used the word perichoresis to describe this kind of oneness. The word perichoresis means mutual indwelling: being-in-one-another. 

Donald Migliore writes, “God is no supreme monad i.e. single unit existing in eternal solitude. God is essentially communal – a communion expressed by the Greek word perichoresis, meaning “mutual indwelling” or “being-in-one-another.” The three of the Trinity “indwell” and pervade each other; they “encircle” each other, being united in an exquisite divine dance, or to use still another metaphor, they ‘make room’ for each other, being incomparably hospitable to each other”.

This mutual indwelling means that we can only think of the three if we also think of the one. It also means that we can only think of the One if we think of the Three.  When we encounter the one person we also encounter the others.  To meet Jesus is to encounter the Father in the Spirit through him.  And, as the Holy Spirit indwells you, Father and Son make their home within you by the indwelling Spirit.  Wherever you have the one you have the three and wherever you have the three you have the one.

All this changes the way we see the Oneness of God.  God’s Oneness isn’t the divine substance that contains three within it.  No!  God’s Oneness is communal. The most intimate, the most loving and the most profound triune communion.  It is the mutual self-giving and mutual interpenetration of the three divine persons.