Pope Paul VI
Sacerdotalis Caelibatus §48
Sacerdotalis Caelibatus: On the Celibacy of the Priest
48 Supported by the power of faith, We express the Church's conviction on this matter. Of this she is certain: if she is prompter and more persevering in her response to grace, if she relies more openly and more fully on its secret but invincible power, if, in short, she bears more exemplary witness to the mystery of Christ, then she will never fall short in the performance of her salvific mission to the world—no matter how much opposition she faces from human ways of thinking or misrepresentations. We must all realize that we can do all things in Him who alone gives strength to souls (97) and increase to His Church. (98)
Source: Sacerdotalis Caelibatus (Vatican.va)