1. Like the fall of Adam and its consequences, Christian life is essentially social. People are God’s adopted children insofar as they are Christians, that is, insofar as they are in Jesus (see Rom 8.9–10, 14–17, 35–39). Thus unity with Jesus is the principle of Christian unity.
2. The Eucharist, however, is primarily the sacrament of unity with Jesus. Therefore, it is also the sacrament by which the unity of the Church is built up and sustained. The Council of Trent stresses the role of the Eucharist as “a symbol of the unity and love” by which Christians are joined (DS 1635/873a). Vatican II teaches that the liturgy, and especially the Eucharist, “daily builds up those who are in the Church, making of them a holy temple of the Lord, a dwelling-place for God in the Spirit (cf. Eph 2.21–22), to the mature measure of the fullness of Christ (cf. Eph 4.13)” (SC 2; Flannery translation).
In his apostolic constitution, Dominicae cenae, John Paul II takes up this theme and clarifies it. The Eucharist builds up the Church (see LG 11). The sacrament involves both fraternal communion of the members of the Church with one another and the communion of Jesus with each participant. The latter is basic, for “in Eucharistic Communion we receive Christ, Christ himself; and our union with him, which is a gift and grace for each individual, brings it about that in him we are also associated in the unity of his Body which is the Church.”16
3. The Eucharist can build up the Church because it establishes a real communion of love, a true union which, nevertheless, leaves distinction intact, between each person and our Lord Jesus. By the holy Sacrament, each of us abides in him and he in each of us (see Jn 6.56). By making us one with him, furthermore, the Eucharist also establishes a bond of communion among us (see Jn 17.20–23). This bond is ecclesial communion; it is constitutive of the Church (see S.t., 3, q. 67, a. 2; q. 73, aa. 2, 4; q. 82, a. 2, ad 3).
4. It might be objected that the preaching of the gospel, faith, and baptism constitute the Church. They do indeed, but only in an initial, incomplete manner, which the Eucharist alone brings to completion. For example, though it is perfectly true that all the baptized are united in Jesus, baptism nevertheless unites Christians by admitting them to the Church, whose inner unity is only fully actualized by their communion in Jesus in the Eucharist (see S.t., 3, q. 63, a. 6; q. 73, a. 3).
5. In considering the manner in which the Eucharist constitutes the Church, one ought not to overlook the union we gain in the Eucharist with the angels and saints in heaven. In Jesus we are united with them; in the Eucharist we share in a hidden way in their glory. Memory of the mother of Jesus and of the other saints and angels in the Eucharist honors them, stimulates our love for them, and so draws us closer to Jesus himself (see LG 48–50).
Baptism is important in the manner of an admission ticket, but it is fulfilled only in the Eucharist to which it admits. In its decree concerning ecumenism, Vatican II teaches: “Baptism, therefore, constitutes a sacramental bond of unity linking all who have been reborn by means of it. But baptism, of itself, is only a beginning, a point of departure, for it is wholly directed toward the acquiring of fullness of life in Christ. Baptism is thus oriented toward a complete profession of faith, a complete incorporation into the system of salvation such as Christ himself willed it to be, and finally, toward a complete participation in Eucharistic communion” (UR 22). Catholics and other Christians share in the same baptism, but the unity of the Church is defective, because Christian communities which lack the sacrament of orders cannot share in the authentic and perfect celebration of the Eucharist.
Someone might object that stress upon the sacramentality of Communion tends to separate the Eucharist as sacrament from the Mass as sacrifice. The answer to this objection is that the consecration of the Mass, which expresses Christ’s redemptive act in the present, is altogether directed to Communion in him: “Take this, all of you, and eat it . . .. Take this, all of you, and drink from it . . ..” At the same time, Communion can unite us with our Lord Jesus in glory and build us up as his body only because Communion really is a sharing in his redemptive sacrifice which reconciles us with the Father.
16. John Paul II, Dominicae cenae, sec. 4, 119–21; 6.