The teacher Pope John Paul II offers a history lesson. The notion of a “Sunday obligation” was not apostolic. It was an early practice that arose organically from a faith-filled people. Only later was it codified:
47. Even if in the earliest times it was not judged necessary to be prescriptive, the Church has not ceased to confirm this obligation of conscience, which rises from the inner need felt so strongly by the Christians of the first centuries. It was only later, faced with the half-heartedness or negligence of some, that the Church had to make explicit the duty to attend Sunday Mass: more often than not, this was done in the form of exhortation, but at times the Church had to resort to specific canonical precepts. This was the case in a number of local Councils from the fourth century onwards (as at the Council of Elvira of 300, which speaks not of an obligation but of penalties after three absences)(Cf. Canon 21, Mansi, Conc. II, 9) and most especially from the sixth century onwards (as at the Council of Agde in 506).(Cf. Canon 47, Mansi, Conc. VIII, 332) These decrees of local Councils led to a universal practice, the obligatory character of which was taken as something quite normal.(Cf. the contrary proposition, condemned by Innocent XI in 1679, concerning the moral obligation to keep the feast-day holy: DS 2152)
Regional councils all cited here: this tells us that bishops and conferences were largely setting the tone when they observed that the people were deficient in practice.
Enter the universal obligation, less than a century old:
The Code of Canon Law of 1917 for the first time gathered this tradition into a universal law.(Canon 1248: “Festis de praecepto diebus Missa audienda est”: Canon 1247, 1: “Dies festi sub praecepto in universa Ecclesia sunt…omnes et singuli dies dominici”.)
The 1983 code reaffirms this obligation:
The present Code reiterates this, saying that “on Sundays and other holy days of obligation the faithful are bound to attend Mass”.(Code of Canon Law, Canon 1247; the Code of Canons of the Eastern Churches, Canon 881, 1, prescribes that “the Christian faithful are bound by the obligation to participate on Sundays and feast days in the Divine Liturgy or, according to the prescriptions or legitimate customs of their own Church sui iuris, in the celebration of the divine praises”.) This legislation has normally been understood as entailing a grave obligation: this is the teaching of the Catechism of the Catholic Church,(No. 2181: “Those who deliberately fail in this obligation commit a grave sin”.) and it is easy to understand why if we keep in mind how vital Sunday is for the Christian life.
It seems that what is required involves more than being unable to celebrate Mass, or even just “missing” Mass. A grave or mortal sin involves the explicit failure to honor the obligation. In other words, an intent to depart from God and the faith community.
The centrality of Sunday is not diminished by what appears to be wiggle room written into canon law. Indeed, the Church has no power to enforce this on earth. And believers, likely too many of them, seem content to stand before God in the afterlife and take a chance their absence from Sunday liturgy is not quite as grave as other sins of the day.
That said, there is no doubt that even when the parish provides poor preaching and music, and extends a stiff unwelcome to believers, people are still missing something vital, something life-giving, when they absent themselves regularly from the Sunday Eucharist. God can certainly work grace in spite of the poverty of liturgical particulars. Part of the Church’s message must also include encouragement for those who have been away a long time. God’s grace is not withheld from sincere seekers, no matter how much they’ve missed Mass in the past.