We believe that the Mass, celebrated by the priest representing the person of Christ by virtue of the power received through the Sacrament of Orders, and offered by him in the name of Christ and the members of His Mystical Body, is the sacrifice of Calvary rendered sacramentally present on our altars. We believe that as the bread and wine consecrated by the Lord at the Last Supper were changed into His body and His blood which were to be offered for us on the cross, likewise the bread and wine consecrated by the priest are changed into the body and blood of Christ enthroned gloriously in heaven, and we believe that the mysterious presence of the Lord, under what continues to appear to our senses as before, is a true, real and substantial presence.
"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."
~ St. Thomas Aquinas.