Mystici Corporis: On the Mystical Body of Christ and Our Union in It With Christ

From the Introduction:

The doctrine of the Mystical Body of Christ, which is the Church, was first taught us by the Redeemer Himself. Illustrating as it does the great and inestimable privilege of our intimate union with so exalted a Head, this doctrine by its sublime dignity invites all those who are drawn by the Holy Spirit to study it, and gives them, in the truths of which it proposes to the mind, a strong incentive to the performance of such good works as are conformable to its teaching. For this reason, We deem it fitting to speak to you on this subject through this Encyclical Letter, developing and explaining above all, those points which concern the Church Militant. To this We are urged not only by the surpassing grandeur of the subject but also by the circumstances of the present time.

2. For We intend to speak of the riches stored up in this Church which Christ purchased with His own Blood, and whose members glory in a thorn-crowned Head. The fact that they thus glory is a striking proof that the greatest joy and exaltation are born only of suffering, and hence that we should rejoice if we partake of the sufferings of Christ, that when His glory shall be revealed we may also be glad with exceeding joy.