How late can a person arrive at Mass, say on a Sunday, in order for that Mass to fulfill that person's obligation to assist at Mass?
While it is true that before the Second Vatican Council some moral theology manuals placed arrival ”before the offertory as the dividing line in deciding whether one fulfilled the Sunday obligation of assistance at Mass.” This is exactly how I was taught in a post Vatican II seminary.
It is true that before the Second Vatican Council some moral theology manuals placed arrival before the offertory as the dividing line in deciding whether one fulfilled the Sunday obligation of assistance at Mass. But after the liturgical reform, with its emphasis on the overall unity of the Mass, modern theologians shy away from such exactitude.
Mass begins with the entrance procession and ends after the final dismissal and we should be there from beginning to end. Each part of the Mass relates and complements the others in a single act of worship even though some parts, such as the consecration, are essential while others are merely important.
To say that there is a particular moment before or after which we are either "out" or "safe," so to speak, is to give the wrong message and hint that, in the long run, some parts of the Mass are really not all that important. It may also give some less fervent souls a yardstick for arriving in a tardy manner. - Communion for Late Arrivals at Mass?
I would like think that if someone who arrives late out of no fault of their own should not be denied Communion.