The soul in its happiness finds itself standing midway in the Embrace and the Kiss of Father and Son. In a manner which exceeds description and thought, the man of God is found worthy to become not God but what God is, that is to say man becomes through grace what God is by nature. (96. Trans. Theodore Berkeley, OCSO)
To me this gets at one of the most critical retrievals we need to make in our contemporary sense of Christianity. The Trinity is not just our doctrine of God; it is our doctrine of the spiritual life. In the most basic terms, our teaching is that God is not some kind of static 'supreme being,' but is constituted in an infinitely dynamic and creative set of relations. What we call our 'spiritual life' is our finding of ourselves drawn into and included in these relations, so that when we take about the procession of the Spirit from the Father and the Son, we aren't talking about some kind of abstract, divine logic, but about the place where we find our own spirits in prayer.
Jesus Christ is conceived by the power of the Holy Spirit, as we say in the creed. He is the stretching forth of the divine relations of the Trinity to include within themselves our humanity. Christianity, then, is the life of being adopted into the eternal Embrace of Father and Son, the enjoyment by grace and adoption of the relationship to God which the human Jesus enjoys by hypostatic union. As William puts it, "man becomes by grace what God is by nature."