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.
The Aquinas Catechism (ed. Sophia Institute Press, 2000) - ISBN: 9781928832102