Apostolic succession, a feature of apostolicity, is commonly understood as a tracing back to the times of the Apostles. It’s almost like tracing back one’s ancestry. Who ordained our Bishop and who ordained the Bishop who ordained our Bishop, and so forth? But, it is more dynamic than looking backward. It is dynamic and forward looking because we deal not with ordinary mundane things. We deal with “Holy Things” and we need apostolic succession to guarantee that an objective action can produce objective result. We want to know that the bread and wine becomes truly the Body and Blood of Christ. Apostolic succession is dynamic because it guarantees that every Catholic present here who receives Holy Communion receives truly the Body and Blood of Christ. Now, perhaps you understand why the General Instruction of the Roman Missal (GIRM) does not allow the priest to simply adlib the Eucharistic Prayer.
Today, we cover the link between the "Eucharist and Ecclesial Communion", [that is, between the Eucharist and “churchy” communion, for want of a better phrase]. It ties in with our Gospel since it is set within the “High Priestly” prayer of Jesus where Jesus prayed for unity amongst His followers. The Church, as such, exists to foster [to nurture] communion or relationship with the Triune God and with one another. For that purpose, she possesses both the Word and the Sacraments, in particular the supreme sacrament of communion or unity, the Eucharist.
If the Eucharist aims at perfecting communion, this means that it cannot be the starting-point for communion. Basically, we do not celebrate the Eucharist in order to create community. Instead, the Eucharist functions to consolidate and perfect our existing union. In other words, the Eucharist is both an expression as well as an intensifier of communion with the Church.
Communion with the Church is both invisible and visible. Since both these dimensions are necessary for celebrating the Eucharist, let me begin by speaking first of the invisible dimension of our communion.
The invisible dimension expresses the unity which in Christ through the working of the Holy Spirit, unites us to the Father and to one another. This invisible dimension of our communion with God presupposes a life of grace expressed through the practice of the virtues of faith, hope and charity. Thus, every Christian who wants to receive Communion must ensure that he or she receives the Eucharist in a worthy manner. There is a strong relationship between Communion and conversion so much so that a person burdened by serious sin needs to confess before going for Communion. Otherwise, he or she would be profaning what he or she receives . This relationship between Communion and conversion is a matter of one’s conscience, a matter between the person and God. Yet, the Church also has the duty to see that those who manifestly refuse to repent should not be given Communion. The fact that there are some Catholics who are not allowed to receive Communion is a particularly sensitive issue especially when it comes to those who live in “objectively immoral” situations, such as invalid marriages. These can arise when a Catholic marries outside the Church without a dispensation or remarries after divorce without an annulment. 
Such a reality that is less than perfect actually reveals the necessity that communion with the Church be visibly expressed. And this communion is expressed through the 3 Cs: Creed, Code and Cult meaning a visible communion in the teaching of the apostles (Creed), in the Church’s hierarchical order (Code) and in the sacraments (Cult). Every Eucharist we celebrate expresses visibly the communion we have with our Bishop and the Pope. In short, visible communion with the Church means that one accepts Catholic doctrine, receives the Church’s sacraments, and is subject to the Church’s governance. The phrase that best describes this visible communion with the Church is to be a practising Catholic.
Visible communion is important and this is one reason why the Church obliges the faithful to attend Sunday Mass and Pastors to make sure that it is possible for every Catholic to do so.
Also, the Eucharist as the Sacrament of Ecclesial Communion has implications for our ecumenical activities. In longing for unity with our separated brothers and sisters, we pray at Mass that God the Father will grant all of us, the fullness of the Holy Spirit so that we may become one Body, one Spirit in Christ. But, since the unity of the Catholic Church with the other Ecclesial Communities is far from perfect, the Catholic Church is cautious in the matter of intercommunion.  For example, a common understanding is that in order to receive Communion, one must believe that it is truly the Body of Christ that one receives. In actual fact, when you receive Communion, you are also saying “Yes, I believe what the Church teaches on Mary, on the Saints. I believe that the Pope is the Vicar of Christ on earth”. When there is no acceptance of the Church’s teachings or governance, then it is a lie to receive Communion. 
We all want to be united. We all long to see visible unity. However, to use Eucharist as the “means” to unity is to short cut the long road to unity and in a way weaken our sense of how far more we are from the goal. Unity is a manifestation of truth. To short cut the long and winding road is to pretend that we are one. John Paul II concludes by saying that “the faithful observance of the body of norms established is a manifestation and a guarantee of our love for Jesus in the Blessed Sacrament, for our brothers and sisters of different Christian confessions—who have a right to our witness to the truth—and it shows our love for the cause of unity”.
 St Paul is quite clear in teaching this in his letter to the Corinthians. “He who receives the Lord unworthily brings condemnation to himself”.
 The challenge in such a situation is “I can’t receive Holy Communion, so why come for Mass”? How does one reconcile our Sunday obligation and the fact that one cannot receive Communion? The Church's solution is found in the practice of “Spiritual Communion”.
 The Pope explains that if we ever give Communion to a baptised non-Catholic, “the intention in so doing is to meet a grave spiritual need for the eternal salvation of an individual believer and not to bring about an intercommunion which remains impossible until the visible bonds of Ecclesial Communion are fully re-established” (see #45). There are conditions to be fulfilled when a situation arises and they are set forth in the Code of Canon Law (CIC). In the case of Eastern Christians (Orthodox Churches) who have valid Eucharists, the Code provides that they may receive reconciliation, Communion, or the anointing of the sick “if they ask on their own for the sacraments and are properly disposed” (Check out CIC 844§2,§3, §4). The sacraments are principal means and signs of Ecclesiastical Communion—expressed through the 3 Cs: creed, code and cult. The other Christians (namely the Protestants) are divided from the catholic Church in varying degrees in faith (creed) worship (cult) and discipline (code) so that a general rule that applies is that the greater the measure of the difference in the creed, code and cult of an Ecclesial Community from that of the catholic Church, the stricter is the application of the restrictions. This is so that “indifferentism” may be avoided.
 Because of the differences in the expression and acceptance of creed, code and cult, free and general sharing of the sacraments would purport to be a sign of something which does not in fact exist. As an aside, the Church’s teaching on pre-marital sex follows along this reasoning too. Marriage between two baptised is a sacrament of Christ’s faithfulness to the Church. Sex before or outside of marriage is to call Christ and the Church to witness to a non-existent reality.