The Catholic Church is holy because it was founded by Jesus Christ, Who is all-holy, and because it teaches holy doctrines according to the will of Christ. It provides the means of leading a holy life, thereby giving holy members to every age.
We can point to the saints as proof that the holiness of Christ is at work in the Catholic Church. But it would be an even greater proof of the holiness of the Church if all of us would live holy lives, if every Catholic were a person of outstanding Christian virtue.
Jesus prayed to His Father for His Church, “Sanctify them in the truth; thy word is truth… And for their sake I consecrate myself, that they also may be consecrated in truth” (John 17:17, 19).
St. Paul reminds us that Jesus Christ “gave himself for us to redeem us from all iniquity and to purify for himself a people of his own who are zealous for good deeds” (Titus 2:14).
The Holy Spirit preserves the Church as the Body of Christ and His Bride, so that in spite of the sins of its members it will never fail in faithfulness to Him and will meet Him in holiness at the end of the world. The Holy Spirit also helps the Church constantly to purify and renew itself.