Every time we commit a mortal sin and are unable to receive the Eucharist because of it, we have (in a certain sense) committed apostasy and separated ourselves from the Body of Christ. This is essentially what the Sacrament of Confession is for --to return in repentance to full communion with the Body (the Church), from which we have separated ourselves by committing mortal sin. And, in the very early days of the Church, this was much more apparent, since committing mortal sin merited automatic, formal excommunication from the Church. But, after A.D. 217, with the Papal relaxation of this original Apostolic discipline and the introduction of the principal of indulgences (literally, the sinless Body of Christ tolerating / “indulging” members who were non-saints, who still struggling to overcome repeated sins), there developed a distinction between true or essential membership in the Church (i.e., living in a state of grace and/or persistent loyalty to Catholic doctrines) and formal or institutional membership in the Church. Even if one is in a state of sin and serious apostasy, one remains an institutional member of the Church until one is either formally excommunicated or one publicly renounces their Catholic faith to embrace another faith (including atheism). In this, the Church of course continues to recognize the “mark” of one’s Baptism, and so always considers the apostate to justly be a member of the Church, albeit a fallen away one. This is why, if one ever wishes to return to the Church (even if they publicly renounced their institutional membership), they need only receive the Sacrament of Confession to be restored to full communion.