The Mystical Body of Christ is a scriptural image of the Church drawn from the teachings of Christ and Saint Paul that illustrates her unity in Christ, her relationship to him, and the interdependence of her members.
The image of the Church as the Body of Christ is found chiefly in the Epistles of Saint Paul as they describe the relationship between the Church and Christ. Saint Paul's understanding derived from his conversion on the road to Damascus, when Christ asked him: "Saul, Saul, why are you persecuting me?... I am Jesus, whom you are persecuting." (Acts of the Apostles 9:4-5; emphasis added). Christ had already completed his earthly ministry; undergone his Passion, Death, and Resurrection; and ascended into Heaven, but he identified the persecuted members of the Church on earth with his own Body. During his public ministry, Christ also expressed this intimate relationship between himself and his Church, where the two are spoken of as one Body. (Cf. CCC 787)
-The Didache Bible
The image of the Church as the Body of Christ is significant because it indicates that the Church is not simply a community of members gathered around Christ, but that the Church is united in him, in his Body. (cf. CCC 789)
The faithful are incorporated and fortified into the Body of Christ through the Sacraments of Initiation, beginning with Baptism. Through the Sacrament of the Eucharist, the faithful are united together in Christ's Mystical Body; that is, brought into communion with one another through communion with Christ, their Head.
(cf. Colossians 1:17-18) (Cf. CCC 789)
As the Mystical Body of Christ, the Church extends Christ's work of salvation throughout time. The faithful play diverse roles in the Church just as various body parts have diverse functions (cf. LG 7). Rather than harm the body, this serves its unity. (Cf. CCC 776, 846, 1111)
The Holy Spirit acts in the Mystical Body of Christ by giving us grace, unifying and animating the Body, comparable to how the soul functions in the human person (cf. LG 7). (Cf. CCC 797,1108)
-The Didache Bible