You can start by looking at the full paragraph:
795 Christ and his Church thus together make up the “whole Christ” (Christus totus). The Church is one with Christ. The saints are acutely aware of this unity:
Let us rejoice then and give thanks that we have become not only Christians, but Christ himself. Do you understand and grasp, brethren, God’s grace toward us? Marvel and rejoice: we have become Christ. For if he is the head, we are the members; he and we together are the whole man. . . . The fullness of Christ then is the head and the members. But what does “head and members” mean? Christ and the Church.230
Our redeemer has shown himself to be one person with the holy Church whom he has taken to himself.231
Head and members form as it were one and the same mystical person.232
A reply of St. Joan of Arc to her judges sums up the faith of the holy doctors and the good sense of the believer: "About Jesus Christ and the Church, I simply know they’re just one thing, and we shouldn’t complicate the matter."233
And the references:
230 St. Augustine, In Jo. ev. 21,8:PL 35,1568.
231 Pope St. Gregory the Great, Moralia in Job, præf.,14:PL 75,525A.
232 St. Thomas Aquinas, STh III,48,2.
233 Acts of the Trial of Joan of Arc.
And read it in the context of the full article (“The Church - Body of Christ”) here: scborromeo.org/ccc/p123a9p2.htm#II