Souls that are in state of grace (without mortal sin) the Mass gives a wonderful force, nurturing to keep them in that state of grace while at the same time canceling the guilt of all venial sins providing that they are repented of them. Saint Augustine clearly says: "he who devoutly hears Holy Mass will receive a great strength to enable him to resist mortal sins and pardon will be given to him for all venial sins which he may have committed up to that hour." The amount we give is what we receive. And this is also true in the Mass. We receive in measure matching the greater or lesser perfection in our disposition, according to how devoutly we assist at Mass. The greater our devotion so much greater will be the benefits resulting from Mass. According to Saint Thomas, all Masses are, as sacraments equal in rank; but they are not equal in effect. It also depends on the celebrant. Therefore, the greater the piety of the priest, so much the greater will be the fruit of the celebration of the Mass.

It is far more important to have more regard to devotion in assisting at Mass than to the number heard, because if you have more devotion in one single Mass than someone else in another fifty Masses, you will give more honor to God in that single Mass and you will take out far greater fruit from it than that other with all his fifty. It is true that through one single Mass attended with perfect devotion it could possibly happen that the justice of God is satisfied for all the wrongdoing of some great sinner. This is in harmony with what the Council of Trent teaches; namely, that by the offering of the holy Sacrifice, God grants the gift of penitence (sorrow) and then by means of pure penitence pardons the most serious and enormous sins. As we are not able to see our internal disposition with which we attend Mass, nor the amount of satisfaction we receive, we should attend as many Masses as we are able to and with as much devotion as possible.

The life of the Venerable Sister Francesca Farnese was at one time troubled by a thousand anxieties of heavenly love. She grieved to see herself covered from head to foot with divine benefits and yet, she was unable to prove in act the gratitude she felt by making to the Lord any fitting return. But one day there is said to have appeared to her the most Holy Virgin who put the heavenly Babe into her arms, "Take Him," she said, "for He is yours because with Him alone you will fulfill all your duties." In the Mass we have the Son of God, not only in our arms but also in our hearts. Neither is there any doubt but that with Him, and Him alone, we shall be able to make up the debt of gratitude which we owe to God. Try to understand how great are our miseries both of body and of the soul and therefore the need we have of the help of God in order that at every moment He may assist and help us, for assuredly He alone is the end and the beginning of all our good, whether here on earth or eternally. What graces, virtues and gifts the Holy Mass calls down on us. It calls down all the spiritual graces, all the goods the soul needs, such as sorrow for sins and victory over temptations whether they are a result from out side trials, like bad companions and demons, or inner, for instance those arising from disobedient desires.
 
The Mass calls down the assistance which is so necessary for enabling us to rise up, to stand upon our feet, to walk forward in the ways of God. It calls down many good and holy ideas and many inner desires which dispose us to shake off luke-warmness and urges us on to work our best with greater dedication with our will more prompt, our intentions more upright and pure; and these bring with them an immeasurable perseverance to never turn our back on Him, because on this depends our eternal salvation and happiness on earth. It calls down temporal (earthly) blessings such as health, abundance and peace. But it does not call down evils, which are their opposites such as pestilences, earthquakes, wars, famines, persecutions, hatreds, calumnies, and injuries. In fact, in the Mass we find liberation from all evils, here we find enrichment by every sort of benefit. In a word, Holy Mass is the golden key of paradise.

Source: St. Leonard of Port Maurice, Hidden Treasure.