Because the Christian God is not a lonely God, but rather a communion of three persons, faith leads human beings into the divine communion. One cannot, however, have a self-enclosed communion with the Triune God- a "foursome," as it were-- for the Christian God is not a private deity. Communion with this God is at once also communion with those others who have entrusted themselves in faith to the same God. Hence one and the same act of faith places a person into a new relationship both with God and with all others who stand in communion with God.
Miroslav Volf, After Our Likeness: The Church as the Image of the Trinity
relationship, trinity