Mutually indwelling June 2012 from Wikipedia

A useful explanation of the relationship of the distinct divine persons is called “perichoresis”, from Greek going around, envelopment. This concept refers for its basis to John 14:17, where Jesus is instructing the disciples concerning the meaning of his departure. His going to the Father, he says, is for their sake; so that he might come to them when the “other comforter” is given to them. Then, he says, his disciples will dwell in him, as he dwells in the Father, and the Father dwells in him, and the Father will dwell in them. This is so, according to the theory of perichoresis, because the persons of the Trinity “reciprocally contain one another, so that one permanently envelopes and is permanently enveloped by, the other whom he yet envelopes”. (Hilary of Poitiers, Concerning the Trinity 3:1).

This co-indwelling may also be helpful in illustrating the Trinitarian conception of salvation. The first doctrinal benefit is that it effectively excludes the idea that God has parts. Trinitarians assert that God is a simple, not an aggregate, being. The second doctrinal benefit is that it harmonizes well with the doctrine that the Christian’s union with the Son in his humanity brings him into union with one who contains in himself, in St. Paul’s words, “all the fullness of deity” and not a part. (See also: Theosis). Perichoresis provides an intuitive figure of what this might mean. The Son, the eternal Word, is from all eternity the dwelling place of God; he is the “Father’s house”, just as the Son dwells in the Father and the Spirit; so that, when the Spirit is “given”, then it happens as Jesus said, “I will not leave you as orphans; for I will come to you.” [John 14:18]

Some forms of human union are considered to be not identical but analogous to the Trinitarian concept, as found for example in Jesus’ words about marriage: “For this cause shall a man leave his father and mother, and cleave to his wife; And they twain shall be one flesh: so then they are no more twain, but one flesh.”[Mark 10:7–8] According to the words of Jesus, married persons are in some sense no longer two, but joined into one. Therefore, Orthodox theologians also see the marriage relationship as an image, or “icon” of the Trinity, relationships of communion in which, in the words of St. Paul, participants are “members one of another”. As with marriage, the unity of the church with Christ is similarly considered in some sense analogous to the unity of the Trinity, following the prayer of Jesus to the Father, for the church, that “they may be one, even as we are one”. [John 17:22]