1. The intimate communion with creation which God initiated in Jesus transcends his relationship with the people of the old covenant: “From his [Jesus’] fulness have we all received, grace upon grace. For the law was given through Moses; grace and truth came through Jesus Christ” (Jn 1.16–17). Jesus pours out his fullness to the Church (see Eph 1.22–23; LG 7), and so in the Church we share in the fullness of Christ (see Col 2.9–13; S.t., 3, q. 8, a. 1; q. 69, a. 5). Human members of God’s family, the Church, are joined with Jesus in three distinct ways.
2. First, the fullness of deity which resides in Jesus in bodily form is communicated to members of the Church by the Holy Spirit. Those who believe in Jesus are begotten by God (see Jn 1.12–17), and this begetting is very real: God gives his gifts that we might be “partakers of the divine nature” (2 Pt 1.4); the word of God, which gives rebirth, is divine semen (see 1 Pt 1.23); the Christian is a child of God, truly of God’s stock (see 1 Jn 2.29–3.1; 3.9). The new People of God enjoy familial intimacy with the Father because through Jesus and in his Spirit we receive the right to commune in divine life, “to become children of God” (Jn 1.12; cf. S.t., 3, q. 8, a. 3; q. 23, aa. 1–2).
3. There is a second profoundly mysterious aspect of the unity between the Lord Jesus and human persons joined with him in the Church: bodily union. The bodies of Christians are members of Jesus (see 1 Cor 6.15). This bond is sacramental but no less physically real on that account (see S.t., 3, q. 8, a. 2; q. 56, a. 1; sup., q. 76, a. 1).
4. A third aspect of the unity between the Lord Jesus and human persons joined with him in the Church is community in human acts. Jesus revealed God’s kingdom by human words and deeds and sought acceptance in faith; on our part, faith requires human acts which are obedient and cooperative (see DV 4–5). Although the gift of the Spirit is distinct from and far transcends the human acts by which we respond to Jesus in faith, our response is nevertheless the condition for receiving the higher unity with Jesus in divine life (see Jn 20.31; S.t., 1–2, q. 108, aa. 1–2).
5. Created persons who make up the Church are fulfilled, not absorbed, by their threefold union with our Lord Jesus (see GS 11, 21–22). In him we are to become one perfect man (see Eph 4.11–13), able to commune with God without ceasing to be the distinct human persons we are. Thus God’s purpose in creating is achieved: His perfection is manifested in humankind fulfilled, which realizes the glory of God.8
Vatican II speaks often of the Church as the “People of God” (see LG 9–17). This expression, rooted in the Old Testament in which Israel is God’s chosen people, can be helpful, but it also suggests the limitation of intimate communion before the fullness of God’s revelation in our Lord Jesus. Another expression, which the Council also uses, is more suggestive of intimacy: “family of God.” The supreme exemplar of the unity of the Church is the divine family, the Trinity (see UR 2). Priests “gather God’s family together as a brotherhood of living unity, and lead it through Christ and in the Spirit to God the Father” (PO 6). In Jesus the human family is called to be the family of God (see GS 32, 40, and 92). The unity of the family of God’s children strengthens and perfects the unity of the human family (see GS 40, 42, and 43).
Because human persons are not naturally children of God, it requires a second birth of the Holy Spirit for them to become so (see Jn 3.3–8). Without at all lessening the realism of the relationship, this rebirth can be considered adoptive incorporation into the divine family; by it we share the image of the eternal Son, and he becomes the firstborn of many brothers (see Rom 8.14–15, 29). God’s adopted children share in his Spirit, whose presence is a pledge of all the divine good to which they are heirs (see Rom 8.14–17; Gal 4.6–7; Eph 1.13–14).
8. Cf. St. Irenaeus, Against Heresies, iv, 20, 7. For the development of the Christian understanding of the unity of the Christian with Jesus, see Emile Mersch, S.J., The Whole Christ: The Historical Development of the Doctrine of the Mystical Body in Scripture and Tradition, trans. John R. Kelly, S.J. (Milwaukee: Bruce, 1938); he gives a specific treatment of St. Irenaeus (227–43), and a fine summary of the synthesis of St. Cyril of Alexandria (337–58) to whose thought the theology presented here is quite close. For a detailed study of some relevant aspects of Cyril’s thought, see Walter J. Burghardt, S.J., The Image of God in Man According to Cyril of Alexandria (Woodstock, Md.: Woodstock College Press, 1957), esp. 105–25. For the scriptural witness to the unity of the Christian with Christ, see also the valuable work of an Anglican scholar, Ernest Best, One Body in Christ: A Study in the Relationship of the Church to Christ in the Epistles of the Apostle Paul (London: S.P.C.K., 1955).