Catechism of the Catholic Church: “The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.” (CCC, n. 2181)
But a substantially-limited deliberate failure in this or any grave obligation is not a mortal sin. Therefore, only those persons who fail to a grave extent to fulfill this grave obligation commit a mortal sin by missing Mass. Attending Mass on Sundays and holy days is only one part of the grave obligation to worship God and to keep holy the Sabbath. The failure to attend Mass, on one particular Sunday or holy day of obligation, is a substantially-limited violation of the positive precepts to worship God and to keep holy the Sabbath. For attending Mass is only one of many ways that we worship God and keep holy the Sabbath.
Missing Mass may be a mortal sin, or a venial sin, or not a sin at all. Missing Mass for a just reason is not a sin at all. The Catechism gives illness and the care of infants as examples of reasons that allow a person to miss Mass without sin. These examples are not grave reasons, but only just reasons. Examples of grave reasons for missing Mass include a severe illness, or the danger of death, or any situation in which grave harm would occur if one attends. But only a just reason (weighty or substantial, but not grave) is needed to miss Mass without sin. Since only a just reason, not a grave reason, is needed to exempt one from the obligation, the matter of missing Mass on one occasion cannot be a grave matter. For only a grave reason excuses one from a grave obligation.
Therefore, while the obligation to worship God, to keep holy the Sabbath, and to attend Mass regularly is a grave obligation, missing Mass on occasion with a just reason is not a sin, and missing Mass on occasion for little or no reason is only a venial sin. The believing and practicing Catholic who regularly attends Mass (on Sundays and holy days), and who worships God and keeps the Sabbath holy in other ways as well, fulfills his grave obligation. If the same Catholic then misses Mass on occasion for little or no reason, his failure is venial; it is not a grave failure that would result in the loss of sanctifying grace, separation from the charity of God, and the punishment of eternal Hellfire.
However, if a Catholic abandons worshiping God or abandons keeping holy the Sabbath, he certainly commits an objective mortal sin. And if a Catholic misses Mass frequently for little or no reason, or if he abandons going to Mass altogether, he certainly commits an objective mortal sin. These are grave failures to fulfill grave obligations, and so the failure is a mortal sin, not merely a venial sin.
Examples: (1) A Catholic usually attends Mass on Sundays and Holy Days of obligation. If he misses Mass on occasion for a just reason, he does not sin at all. Just reasons for missing Mass include illness, caring for someone else who is ill, caring for young children, requirements of work, the difficulty of getting to Mass while traveling, or reasons of similar weight.
(2) The same Catholic misses Mass on one occasion for little or no reason. He oversleeps, or he does not feel motivated to attend Mass, or he has a social event he wishes to attend, or he wishes to watch a sports event. His reason for missing Mass is not a just reason; these and other similar reasons lack sufficient moral weight. His failure to attend Mass on such an occasion is a venial sin.
(3) A Catholic decides that he is no longer going to practice the Catholic Faith by attending Mass, or that he is only going to attend Mass on special occasions, such as Christmas, Easter, and the weddings and funerals of family members. Such a substantial failure to fulfill the grave obligation to attend Mass is a mortal sin.
For more on this topic, see The Catechism of Catholic Ethics, n. 113.
Ronald L. Conte Jr.
Roman Catholic theologian and Bible translator