From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
Following Christ is not an outward imitation, since it touches man at the very depths of his being. Being a follower of Christ means becoming conformed to him who became a servant even to giving himself on the Cross (cf. Phil 2:5-8). Christ dwells by faith in the heart of the believer (cf. Eph 3:17), and thus the disciple is conformed to the Lord. This is the effect of grace, of the active presence of the Holy Spirit in us.
Having become one with Christ, the Christian becomes a member of his Body, which is the Church (cf. Cor 12:13, 27). By the work of the Spirit, Baptism radically configures the faithful to Christ in the Paschal Mystery of death and resurrection; it “clothes him” in Christ (cf. Gal 3:27): “Let us rejoice and give thanks”, exclaims Saint Augustine speaking to the baptized, “for we have become not only Christians, but Christ (. . .). Marvel and rejoice: we have become Christ!”28 Having died to sin, those who are baptized receive new life (cf. Rom 6:3-11): alive for God in Christ Jesus, they are called to walk by the Spirit and to manifest the Spirit’s fruits in their lives (cf. Gal 5:16-25). Sharing in the Eucharist, the sacrament of the New Covenant (cf. 1 Cor 11:23-29), is the culmination of our assimilation to Christ, the source of “eternal life” (cf. Jn 6:51-58), the source and power of that complete gift of self, which Jesus — according to the testimony handed on by Paul — commands us to commemorate in liturgy and in life: “As often as you eat this bread and drink the cup, you proclaim the Lord’s death until he comes” (1 Cor 11:26).
28 In Iohannis Evangelium Tractatus, 21, 8: Corpus Christianorum, Latin series 36, 216.
— Veritatis Splendor 21