14 To this it may be added that the expiatory passion of Christ is renewed and in a manner continued and fulfilled in His mystical body, which is the Church. For, to use once more the words of St. Augustine, "Christ suffered whatever it behoved Him to suffer; now nothing is wanting of the measure of the sufferings. Therefore the sufferings were fulfilled, but in the head; there were yet remaining the sufferings of Christ in His body" (In Psalm lxxxvi). This, indeed, Our Lord Jesus Himself vouchsafed to explain when, speaking to Saul, "as yet breathing out threatenings and slaughter" ( Acts ix, 1), He said, "I am Jesus whom thou persecutest" ( Acts ix, 5), clearly signifying that when persecutions are stirred up against the Church, the Divine Head of the Church is Himself attacked and troubled. Rightly, therefore, does Christ, still suffering in His mystical body, desire to have us partakers of His expiation, and this is also demanded by our intimate union with Him, for since we are "the body of Christ and members of member" (1 Corinthians xii, 27), whatever the head suffers, all the members must suffer with it (Cf. 1 Corinthians xii, 26).
Source: Miserentissimus Redemptor (Vatican.va)