The Nature of our Present Union
Present union is also spiritual. This is so not in an immaterial, idealistic sense but because of the activity and indwelling of the Holy Spirit. This gives to present union with Christ its distinctiveness. It also circumscribes the mystery involved and protects against confusing it with other kinds of union. As spiritual, that is, effected by the Holy Spirit, it is neither ontological (like that between the persons of the Trinity), nor hypostatic (like that between Christ’s divine and human natures), nor psychosomatic (between body and soul in human personality), nor somatic (between husband and wife); nor is it merely intellectual and moral, a unity in understanding, affections and purpose. Spiritual union stems from the relationship between Christ and the Holy Spirit given with his glorification and lying in back of that union.
Richard Gaffin, By Faith, Not by Sight: Paul and the Order of Salvation (Paternoster, 2006), 38-39.