What does obligation mean
Re: At what point in Mass does a later arrival not fulfill the Mass obligation?
First let's distinguish between the two different questions: The first question, to which you referred and which I answered, is "Can you receive Communion if you are late for Mass?" The second question, your question, is "At what point in the Mass does one's later arrival mean that one has not fulfilled a Mass obligation on a Sunday or holy day?" This thread will deal with the second question.
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. Although I am a post-Vatican-II convert myself, I have been told by pre-Vatican-II Catholics that 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 hazards slowed down traffic; 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.
Basically, the Church wants us to be there for the full Mass:
§1 The obligation of assisting at Mass is satisfied wherever Mass is celebrated in a Catholic rite either on a holy day itself or on the evening of the previous day.Source: forums.catholic.com