In its widest sense, the phrase union with Christ refers to the relationship between the believer and Jesus Christ. In this sense, John Murray says that union with Christ is "the central truth of the whole doctrine of salvation". The expression "in Christ" (en Christo, en kyrio, en Christo Iesou, en auto etc.) appears frequently in the New Testament: according to Albert Schweitzer, "'being-in-Christ' is the prime enigma of the Pauline teaching: once grasped it gives the clue to the whole." Given the large number of occurrences and the wide range of contexts, the phrase embodies a breadth of meaning.
According to the narrower sense of the phrase used in Christian theology, union with Christ is a step in the ordo salutis ("order of salvation"), and the basis of the believer's justification. In this sense, union with Christ follows faith and precedes adoption (Galatians 3:26-27). Augustus Strong describes it in this way: "union with Christ logically precedes both regeneration and justification; and yet, chronologically, the moment of our union with Christ is also the moment when we are regenerated and justified."
