At what point in Mass does a late arrival not fulfill the Mass obligation?
Prior to Vatican II, the common catechesis was that a person had to be present for the Offertory, Consecration, and Communion or one had not fulfilled the Mass obligation and was required to go to another Mass. While a noble attempt to get people to church on time by giving them the time at which they were late, it had two unforeseen effects:
• Those with freer consciences would arrive after the start of the Mass, knowing that as long as they got there on time for the Offertory all was well. It was not unusual for people to walk in after the Mass began but before the Offertory started.
• Those with tender consciences suffered deeply from scruples and would believe themselves in a state of mortal sin even though their tardiness to Mass was entirely out of their control (e.g. the car broke down; road accident that caused traffic to be delayed, etc.)
Both of these conditions were unhealthy, and following Vatican II the cut-off point of the Offertory was dropped. Another reason that contributed to that was the elevation of the Liturgy of the Word and the homily to their modern importance in the Mass.
If there is just cause for being late to a particular Mass, one has still met one’s Sunday obligation (and can receive Communion), but being later should not become a habit. If there is not just cause, one may still have met the Sunday obligation but the fact that one has not treated the Mass as a serious and holy event to which one should be prompt might be a matter to consider during an examination of conscience. If the matter is not mortally sinful because of lack of full knowledge or lack of free consent, one can still receive Communion. Because there is no longer a cut-off point after which you are late for Mass, the temptation to regularly budget one’s time around the Offertory and slide into the pew just as it begins is removed.
When considering the question, our primary concern should be to be present for the re-presentation of Christ’s sacrifice on Calvary. Those who were present that day two thousand years ago gathered around their Lord as quickly as they could and stayed with him throughout his agony, praying and suffering as well. Two thousand years later, we owe our Lord no less devotion than our forefathers and foremothers in the faith gave him then.