Union with Christ

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, 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.) occurs 216 times in the Pauline letters and 26 times in the Johannine literature. Hence, according to Albert Schweitzer, "This '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. (For example; Paul uses the phrase "en Christo" as a synonym for a Christian (Romans 16:7) and the phrase "en emoi" to describe the intimate identification of Christ with the believer (Galatians 2:20).)

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."

Traditional Roman Catholic theology centres the union with Christ in a substantial sense on the unity of the institutional church, past and present. "The communion of saints is the spiritual solidarity which binds together the faithful on earth, the souls in purgatory, and the saints in heaven in the organic unity of the same mystical body under Christ its head." Christians, according to Roman Catholic theology, are united to Christ through the sacraments.

...
Wikipedia