By penitence and confession…

Like hairs on the head, mortal man is joined to Jesus Christ, the head of all, but they are full of transgressions and sins because of man’s delight in the flesh. But the Church regenerates and purifies these from the unclean stench and filth of sin by penitence and confession, just as hair is cleansed from dew and drops, and as dust is shaken out and cleansed from wool.
–Saint Hildegarde of Bingen (1098-1179)

The indestructible church…

The Church has ever proved indestructible. Her persecutors have failed to destroy her; in fact, it was during times of persecution that the Church grew more and more; while the persecutors themselves, and those whom the Church would destroy, are the very ones who came to nothing. . . .Again, errors have assailed her; but in fact, the greater the number of errors that have arisen, the more has the truth been made manifest. . . . Nor has the Church failed before the assaults of demons: for she is like a tower of refuge to all who fight against the Devil.
–Saint Thomas Aquinas (1225-1274)

The Catholic Church is…

Just as in one man there is one soul and one body, yet many members; even so the Catholic Church is one body, having many members. The soul that quickens this body is the Holy Spirit; and therefore in the Creed after confessing our belief in the Holy Spirit, we are bid to believe in the Holy Catholic Church.
–Saint Thomas Aquinas (1225-1274)

Grace in the sacrament…

In the life of the body a man is sometimes sick, and unless he takes medicine, he will die. Even so in the spiritual life a man is sick on account of sin. For that reason he needs medicine so that he may be restored to health; and this grace is bestowed in the Sacrament of Penance.
–Saint Thomas Aquinas (1225-1274)