by Fr. Cecilio Magsino
In the Last Supper, Jesus prayed for the unity of his disciples. In his prayer to God the Father he beseeched, “May they all be one. Father, may they be one in us, as you are in me and I am in you, so that the world may believe it was you who sent me. I have given them the glory you gave to me, that they may be one as we are one.” (Jn 17:21-22)
In this article I reflect on how the Catholic Church understands unity based on some passages of Sacred Scripture and the Fathers of the Church. Understanding unity this way can guide how we might live as Catholics in our relations with fellow Catholics and with other non-Catholic Christians.
We begin by recalling that in 381 AD the First Council of Constantinople professed a Creed which proclaimed our belief in the one, holy, catholic, and apostolic Church: (Πιστεύομεν) Εἰς μίαν ἁγίαν καθολικὴν καὶ ἀποστολικὴν ἐκκλησίαν· This is translated as “We believe in one, holy, catholic, and apostolic Church.” We can appreciate that very early in the history of the Church, she understood that the Church is one. Unity is an essential feature of the Church. Belief in her unity is part of the faith as expressed in the Creed of Constantinople.
What is the meaning of this “unity”?
Unity is a very difficult concept to define. A correct definition should not use the same word or concept in the definition. Webster defines unity as “the quality or state of being one.” That is not a good definition because we use the same concept to define it. Another definition is “the state or quality of not being multiple.” This is not a good definition either because we cannot define something by saying what it is not (like defining black by saying it is not white, or yellow, or green, etc.) Unity or oneness is what philosophers call a “transcendental property of being.” Any being is one. As such we simply have an intuition or understanding of it and it really is “undefinable” because to define something we must find the “genus” the encompasses the thing we want to define and then give its specific difference. We might define man as “a rational animal.” Animal is the genus that will include man in it and rational is man’s specific difference. But to define unity we cannot find a genus that will be “higher” that the genus one belongs to because one is a property of all being and there is no genus higher that being.
If we regard the Church as the “assembly” established by Jesus, and this assembly is a “being” then it must have the transcendental property of being one. Now the question is “which is this assembly?” Is it the group of all the people who believe in Jesus Christ? Is it the society of people who call themselves Christians? Is it the people who believe in the New Testament? Or the people who practice the same sacraments?
The Teaching of Jesus
Let’s begin our effort to understand the unity of the Church by going back to the Jesus’ teachings, which is where our entire faith is founded.
Jesus taught many deep and fundamental truths using parables. Many of them were about what he called the “kingdom of heaven” (the expression found in the Gospel according to St. Matthew) or the “kingdom of God” (the expression in St. Mark and St. Luke). Both expressions can be understood to mean the people over whom heaven reigns or God is king. By “kingdom of God” Jesus was referring to an assembly in the here and now present, perfectly realized at the end of history. He was talking about the Church.
If we were listening directly to Jesus as he spoke about the kingdom of heaven, or the kingdom of God, what would be very clear to us is that he was speaking of a kingdom, that is, one kingdom and not many kingdoms.
From the parable of the sower: “The kingdom of heaven may be compared to a man who sowed good seed in his field.”
From the parable of the mustard seed: “The kingdom of heaven is like a mustard seed which a man took and sowed in his field.”
From the parable of the yeast: “The kingdom of heaven is like the yeast a woman took and mixed in with three measures of flour till it was leavened all through.”
The Church is a very deep mystery and so Jesus used many more images to describe or define it: town, household, city, and flock. In each of these images Jesus always spoke of the kingdom of heaven or of God and its unity. Such unity is by necessity for the kingdom’s survival: Jesus said that “every kingdom divided against itself is heading for ruin; and no town, no household divided against itself can stand.” The Church, Jesus implies, is a united “city built on a hilltop that cannot be hidden.” Jesus, being the Good Shepherd who takes care of his Sheep, will lead a united flock. He will lead as well those that he has but are not of the same fold, and all will listen to his voice, to the point that there will be only one flock, and one shepherd. .”
The Catholic Church understands that she herself is this flock. We recall that Jesus had entrusted his flock of sheep to Peter after his resurrection. And after Peter, the Shepherd Jesus’ flock of sheep are the successors of Peter in this ministry which is the Petrine ministry. Here we can appreciate the role of Peter (and his successors) in keeping and promoting the unity of the Church. Being under the care of Peter and belonging to his flock is a sign of the unity of the Church. This Church Jesus founded is the Roman Catholic Church because it is the only Church that can claim the continuity of successors of Peter.
The existence, therefore, of many churches that do not belong to the care of the successor of Peter contradicts the will of Jesus that there be one flock under one Shepherd. There is an effort and movement within the Catholic Church to bring all Christians to the unity that is the foundational will of Jesus. This movement is call Ecumenism.
Causes of Disunity
To understand better what the unity of the Church means, we can look at what Scripture also mentions about those things that cause the disunity of the Church. For this we must go to the letters of St. Paul. In his letter to the Galatians, he wrote,
When self-indulgence is at work the results are obvious: fornication, gross indecency, and sexual irresponsibility; idolatry and sorcery; feuds and wrangling, jealousy, bad temper, and quarrels; disagreements, factions, envy; drunkenness, orgies and similar things. I warn you now, as I warned you before: those who behave like this will not inherit the kingdom of God.
He enumerated these grave sins that would cause the unrepentant and guilty person to lose the kingdom of God. In other words, sin is what will cause one to be excluded from the kingdom in the next life definitively. In the present life, those guilty of these sins do cause a division in the Church. What disunites is sin. The opposite, what unites, is love for God and neighbor.
St. Paul was very aware that there were factions in the nascent Church. He warned Christians to avoid those deeds and attitudes that could cause those divisions and so that they preserve their unity. In his first letter to the Corinthians, he referred to these dissensions and divisions present in that church:
All the same, I do appeal to you, brothers, for the sake of our Lord Jesus Christ, to make up the differences between you, and instead of disagreeing among yourselves, to be united again in your belief and practice. From what Chloe’s people have been telling me, my dear brothers, it is clear that there are serious differences among you.
What I mean is that each one of you says, ‘I belong to Paul,’ or ‘I belong to Apollos,’ or ‘I belong to Cephas,’ or ‘I belong to Christ.’ Is Christ divided? Was Paul crucified for you? Or were you baptized in the name of Paul?
The divisions among the Christians of Corinth contradicted what Paul knew to be the Church, the mystical body of Christ and so he asked the rhetorical question furthermore, Is Christ divided?.
We recall that St. Paul learned the hard way that Christians and Christ are somehow one in a mysterious way. On his way to Damascus to arrest some Christians, suddenly a light appeared, and Paul fell to the ground. Jesus appeared to him. “Then he heard a voice saying, ‘Saul, Saul, why are you persecuting me?’ ‘Who are you, Lord?’ he asked, and the voice answered, ‘I am Jesus, and you are persecuting me.”
Since the body of Christ is one and not dismembered, so then the Church, the Christians who are members of the Body of Christ must be likewise one and not dismembered.
It is interesting to note one source of disunity Paul pointed out: the preaching of a gospel that is different from what the apostles have preached. In his letter to the Galatians he wrote,
I am astonished at the promptness with which you have turned away from the one who called you and have decided to follow a different version of the Good News. Not that there can be more than one Good News; it is merely that some troublemakers among you want to change the Good News of Christ; and let me warn you that if anyone preaches a version of the Good News different from the one we have already preached to you, whether it be ourselves or an angel from heaven, he is to be condemned.
The Struggle for Unity
From the beginning of its history, the Church has been zealous to keep the purity of her faith. She has marked out and rejected heresies, erroneous understandings of the teachings of Jesus and the apostles. Faith and belief matter a lot. Keeping the faith means clinging to the truth. Falsehood and error are sources and causes of division. The truth can only be one because the source of all truth is only one and that is God.
The early Church had to struggle against heresies about Jesus and the Blessed Trinity. Examples of heresies about Jesus Christ are Docetism, Monophysitism, and Arianism among others. Let’s briefly revisit these heresies and how the Church opposed them.
Docetism taught that Jesus just seemed to have a human body, but he really did not have one. Jesus, for them, was not true man. This heresy thinks that God cannot become man so that he remains God. If he became man, then he just appears to be a man. St. Ignatius of Antioch challenged it in the 2nd century. The first council of the Church, the Council of Nicaea (325) rejected Docetism.
Monophysitism claimed that Jesus had only the divine nature. Some form of monphysitism claimed that the divine and human nature of Jesus was merged into one. Monos in Greek means one and physis means nature. The Council of Chalcedon (451) decreed that Jesus is one person with two natures, the divine and the human.
Arianism is the heresy of Arius, a priest of Alexandria who preached this doctrine towards the end of the 3rd century. He thought the God is only one and that there can be only one Person is God. He is the Father. Jesus is just a creature and not true God, although he was endowed with some powers. This doctrine provoked some disturbance within the Church and the Council of Nicaea was convened to settle it. The Council declared Arius to be wrong and defined that Jesus is from the same substance as the Father (εκ τής ούσίας τοΰ πατρός, in the words of the Creed of Nicaea). Jesus is God as the Father is.
The Early Fathers of the Church and Unity
Because of the doctrinal and disciplinary issues faced by the early Christians, the early Fathers of the Church sought to teach and to work out the unity of the Christians. I will just bring up a couple of examples of their teaching.
St. Clement of Rome wrote a letter to the Corinthians sometime in the year 70 AD. He devoted a short chapter to remind the Christians of Corinth to be united among themselves, and to be vigilant against disunity, its causes and consequences:
To such examples ought we also to cleave, brethren. For it is written, “Cleave unto them that are holy, for they that cleave unto them shall be made holy.” And again, in another place he saith, “With the guiltless man thou shalt be guiltless, and with the excellent thou shalt be excellent, and with him that is crooked thou shalt be perverse.” Let us, therefore, cleave to the guiltless and the just, for they are the elect of God. Why are there strivings, and anger, and division, and war among you? Have we not one God and one Christ? Is not the Spirit of grace, which was poured out upon us, one? Is not our calling one in Christ? Why do we tear apart and rend asunder the members of Christ, and make sedition against our body, and come to such a degree of madness that we forget we are members one of another? Remember the words of our Lord Jesus, for he said, “Woe unto that man; it were good for him if he had never been born, rather than that he should cause one of my elect to offend. It were better for him that a millstone were tied about him, and that he were cast into the sea, rather than that he should cause one of my little ones to offend.” This your schism has perverted many; hath cast many into despondency; many into doubt; all of us into grief, and, as yet, your sedition remaineth.
St. Ignatius of Antioch, on the other hand, wrote a letter to the Christians of Philadelphia while he was being transported from Antioch to Rome to be martyred around the year 110 AD. This is how he began Chapter 3 of his letter:
Keep yourselves from those evil plants which Jesus Christ does not tend, because they are not the planting of the Father. Not that I have found any division among you, but exceeding purity. For as many as are of God and of Jesus Christ are also with the bishop. And as many as shall, in the exercise of repentance, return into the unity of the Church, these, too, shall belong to God, that they may live according to Jesus Christ. Do not err, my brethren. If any man follows him that makes a schism in the Church, he shall not inherit the kingdom of God. If anyone walks according to a strange opinion, he agrees not with the passion of Christ.
Working towards Unity: Concluding thoughts
The Church is one and united. She is one because she professes the same faith; she practices the same sacraments, especially the Eucharist. More importantly, the Church is the one flock under the leadership of the one shepherd who is the Pope.
The Church is one despite the diversity in peoples and cultures that comprise it. Historically, the way the Catholics have worshiped has been diverse and so there are many different families of liturgical traditions. And yet all constitute one Church.
What is decisive for the unity of the Church is unity in faith (doctrine, and consequently moral unity among Christians), unity in the sacraments, and finally unity with the Holy Father. If we safeguard these elements of unity we preserve the unity of the Church as Jesus wants it.
 See for example https://www.fourthcentury.com/constantinople-381-creed-greek-and-english/.
 Although “heaven” and “God” are different words and concepts in this context they mean the same. What is heaven? We usually understand it to mean “the place where God is or dwells.” Philosophically, however, God cannot be in a place because place requires matter both in the place and the thing that is in a place. But God is spirit and not material and so God is not in a place. Another definition of heaven is to be with God or to live with God or to share in the life of God. In this way heaven is the same “thing” as God.
 Fradd, M., What is the Kingdom of God? | Catholic Answers.
 About the expression “kingdom of heaven” or “kingdom of God” as referring to the Church see for example Franz Maria Moschner, The Kingdom of Heaven in Parables. Herder, London: 1955.
 Mt 13:24
 Mt 13:31
 Mt 13:33
 Mt 12:25
 Mt 5:14
 Cf Jn 10:16
 After Jesus’ resurrection, he appeared to the disciples at the Sea of Galilee. He performed again the miraculous catch of fish. Jesus gave the disciples breakfast. And this is the account of what followed. “After the meal Jesus said to Simon Peter, ‘Simon son of John, do you love me more than these others do?’ He answered, ‘Yes Lord, you know I love you’. Jesus said to him, ‘Feed my lambs’. A second time he said to him, ‘Simon son of John, do you love me?’ He replied, ‘Yes, Lord, you know I love you’. Jesus said to him, ‘Look after my sheep’. Then he said to him a third time, ‘Simon son of John, do you love me?’ Peter was upset that he asked him the third time, ‘Do you love me?’ and said, ‘Lord, you know everything; you know I love you’. Jesus said to him, ‘Feed my sheep.” (Jn 21:15-17)
 1 Cor 1:10-11
 1 Cor 1: 12-13 (Revised Standard Version Catholic Edition)
 Acts 9:4-5
 Gal 1:6-8
 See https://www.britannica.com/topic/Docetism.
 See https://www.britannica.com/topic/monophysite
 Denzinger, Henrico. Enchiridion Symbolorum Definitionum et Declarationum de Rebus Fidei er Morum, London: Herder, 1911, p. 30.
 Chapter 46 of the letter. It can be accessed from several sites on the Internet, one example is https://www.earlychristianwritings.com/text/1clement-hoole.html#:~:text=0%3A1%20The%20Church%20of,Almighty%20God%20through%20Jesus%20Christ.
 Chapter 3 of the letter. It can be accessed from several places on the Internet, one example is http://www.earlychristianwritings.com/text/ignatius-philadelphians-longer.html.