Unity quotes from throughout Christian history.
Ignatius, A.D. 110
It is fitting that you should glorify Jesus Christ, who has glorified you, in every way, so that by a unanimous obedience you may be perfectly joined together in the same mind and in the same judgment and may all speak the same thing concerning the same thing, and so that, as you are subject to the bishop and the elders, you may be made holy in every way. (Letter to the Ephesians 2)
Ignatius and Unity
Ignatius can be a very difficult person for modern non-Catholics to read. In fact, he stands out as the lone 2nd century writer with such an emphasis on clergy.
This is easily explained. Ignatius is so early that the gnostics were still in the church. Gnostic teachers would open schools, teaching that Jesus was opposed to the God of Israel and came to replace him as well as other ridiculous notions, and they would do so as representing the church.
Ignatius was trying to put a stop to this. So he called Christians to stay united with their bishop to whom the faith was committed by the apostles.
This is addressed more thoroughly on my page on Ignatius, a great man and martyr for Christ.
Let no man deceive himself, if anyone is not within the altar, he is deprived of the bread of God. For if the prayer of one or two possesses such power, how much more that of the bishop and the whole church. He, therefore, that does not assemble with the church has even by this manifested his pride and condemned himself. (Letter to the Ephesians 5)
Take heed to come together often to give thanks to God and show forth his praise. For when you assemble frequently in the same place, the powers of Satan are destroyed and the destruction at which he aims is prevented by the unity of your faith. Nothing is more precious than peace, by which all war, both in heaven and earth, is brought to an end. (Letter to the Ephesians 13)
Do not err, my brethren. Those that corrupt families shall not inherit the kingdom of God. If, then, those who do this in regard to the flesh have suffered death, how much more shall this be the case with any one who corrupts by wicked doctrine the faith of God, for which Jesus Christ was crucified! Such a one becomes defiled and shall go away into everlasting fire. So shall every one that listens to him. (Letter to the Ephesians 16)
Come together in common and individually—through grace, in one faith of God the Father, and of Jesus Christ His only-begotten Son … under the guidance of the Comforter, in obedience to the bishop and the elders with an undivided mind. Break one and the same bread, which is the medicine of immortality, the antidote which prevents us from dying, and a cleansing remedy driving away evil so that we should live in God through Jesus Christ. (Letter to the Ephesians 20)
As the Lord did nothing without the Father … so you should not do anything without the bishop and elders. Do not try to make anything seem reasonable and proper to yourselves separate from others. Instead, when you come together into the same place, let there be one prayer, one supplication, one mind, and one hope in love and joy undefiled. There is one Jesus Christ, and nothing is more excellent than him. Therefore, all of you run together as into one temple of God, as to one altar, as to one Jesus Christ, who came from one Father and is with and has gone to that same One. (Letter to the Magnesians 7)
He that is within the altar is pure, but he that is outside is not pure. That is, he who does anything apart from the overseer, elders, and deacons—such a man is not pure in his conscience. (Letter to the Trallians 7)
Take heed, then, to have but one Eucharist. For there is one flesh of our Lord Jesus Christ, and one cup for the unity of his blood; one altar; as there is one bishop, along with the elders and deacons [lit. servants], my fellow-servants. (Letter to the Philadelphians 4)
Avoid all divisions as the beginning of evils. (Letter to the Smyrneans 7)
The Didache, A.D. 80 - 160
My child, night and day remember him that speaks the Word of God to you, and thou shalt honour him as the Lord. For in the place where lordly rule is uttered, there is the Lord. And you shall seek out the faces of the saints daily, so that you may rest upon their words. (ch. 4)
You shall not long for division, but shall bring those who contend to peace. (ch. 4)
Irenaeus, c. A.D. 185
The Church, having received this preaching and this faith, although scattered throughout the whole world, yet, as if occupying one house, carefully preserves it. She believes these things … and she proclaims them, teaches them, and hands them down with perfect harmony, as if she possessed only one mouth. (Against Heresies, I:10:2)
The preaching of the church is everywhere consistent and continues on an even course. It receives testimony from the prophets, the apostles, and all the disciples ... through the beginning, the middle, the end, and through the entire dispensation of God. That well-grounded system which tends to man's salvation, namely, our faith, which we have received from the church, we preserve. It is always, by the Spirit of God, renewing its youth, as if it were some precious deposit in an excellent vessel causing even the vessel that contains it to renew its youth as well. For this gift of God has been entrusted to the Church, as breath was to the first created man, for this purpose: that all the members receiving it may be brought to life. The fellowship with Christ has been distributed throughout it, that is, the Holy Spirit, the earnest of incorruption, the means of confirming our faith, and the ladder of ascent to God.
"For in the Church," it is said, "God has set apostles, prophets, teachers" [Eph. 4:11] and all the other means through which the Spirit works, of which all those do not partake who do not join themselves to the church, but defraud themselves of life through their perverse opinions and infamous behavior. For where the church is, there is the Spirit of God, and where the Spirit of God is, there is the church and every kind of grace, but the Spirit is truth. Those, therefore, who do not partake of him are neither nourished into life from the mother's breasts, nor do they enjoy that most clear fountain which issues from the body of Christ. Instead, they dig for themselves broken cisterns out of earthly trenches and drink putrid water out of mire, fleeing from the faith of the Church lest they be convicted, rejecting the Spirit so that they may not be instructed. (Against Heresies III:24:1)
Tertullian, c. A.D. 210
Is it likely that so many churches, and they so great, should have gone astray into one and the same faith? No casualty distributed among many men issues in one and the same result. Error of doctrine in the churches must necessarily have produced various issues. When, however, that which is deposited among many is found to be one and the same, it is not the result of error, but of tradition. Can anyone, then, be reckless enough to say that they were in error who handed on the tradition? (Prescription Against Heretics 28)
... there is one Church, divided by Christ throughout the whole world into many members, and also one episcopate diffused through a harmonious multitude of many bishops. (Letters of Cyprian 51:24)
Dionysius of Alexandria, A.D. 251
This letter is to Novatian, who split the church in Rome in 251 by having himself elected a second bishop.
Dionysius to his brother Novatus, greeting. If, as you say, you have been led on unwillingly, you will prove this if you retire willingly. It would be better to suffer everything rather than divide the Church of God. Even martyrdom for the sake of preventing division would not be less glorious than for refusing to worship idols. Nay, to me it seems greater. For in the one case a man suffers martyrdom for the sake of his own soul; in the other case on behalf of the entire Church. Now if you can persuade or induce the brothers to come to unanimity, your righteousness will be greater than your error, and this [error] will not be counted, but that [righteousness] will be praised. But if you cannot prevail with the disobedient, at least save your own soul. I pray that you may fare well, maintaining peace in the Lord. (Eusebius. Church History. Bk. 6, ch. 45, par. 2)
Ephraim the Syrian, d. 373-378
I found out that the Son was incomprehensible, since he is wholly light. In his brightness I beheld the Bright One who cannot be clouded and in his pureness a great mystery, that is, the body of our Lord, which is well-refined. In its undividedness I saw the Truth that is undivided. (The Pearl: Seven Hymns on the Faith. Hymn 1.)
Vincent of Lerins, d. A.D. 445
We come now to the Vincentian Canon: “In the Catholic Church itself, every care should be taken to hold fast to what has been believed everywhere, always, and by all” (Commonitories 2 [p. 270]). This sentence is often shortened into the phrase “everywhere, always, and by all.” If you want good theology, look to the ancient Catholic Church, spread throughout the world and continuous over time. Any point of doctrine that clearly doesn’t meet the standards of the Canon—having been believed everywhere, always, and by all— should be treated with great suspicion. If the doctrine in question was part of original Christian teaching, why was it not preserved by the historic Catholic Church? (Doherty, D. "An Ancient Strategy for Rooting Out Bad Theology." Web. Echoes in the Catacombs. June 4, 2018.)
Jerome, c. A.D. 379
We ought to remain in that Church which was founded by the apostles and continues to this day. If ever you hear of any that are called Christians taking their name not from the Lord Jesus Christ, but from some other—for instance, Marcionites, Valentinians, Men of the Mountain or the Plain—you may be sure that there you do not have the Church of Christ, but the synagogue of Antichrist.
For the fact that they took their rise after the foundation of the Church is proof that they are those whose coming the apostle foretold. And let them not flatter themselves if they think they have Scripture authority for their assertions; since, the devil himself quoted Scripture, and the essence of the Scriptures is not the letter, but the meaning. Otherwise, if we follow the letter, we too can concoct a new dogma and assert that such persons as wear shoes and have two coats must not be received into the Church. (Dialogue Against the Luciferians 28)
John Calvin, 1540 - 1564
The only true bond of Church unity is Christ the Lord, who has reconciled us to God the Father, and will gather us out of our present dispersion into the fellowship of His body, that so, through His one Word and Spirit, we may grow together into one heart and one soul. (Letter to Cardinal Sadolet, from Philip Schaff, History of the Christian Church, vol. VIII, sec. 91; note that this may be vol. VII in the print version; my electronic version was obtained from the Christian Classics Ethereal Library)
Let these two points, then, be considered as decided: first, that he who voluntarily deserts the external communion of the Church where the Word of God is preached, and the sacraments are administered, is without any excuse; secondly, that the faults either of few persons or of many form no obstacles to a due profession of our faith in the use of the ceremonies instituted by God; because the pious conscience is not wounded by the unworthiness of any other individual, whether he be a pastor or a private person; nor are the mysteries less pure and salutary to a holy and upright man, because they are received at the same time by the impure. (Institutes of the Christian Religion, Book IV, as cited by Schaff, ibid., bk. VIII, ch. 13, sec. 98)
We are as ready to confess as they are that those who abandon the Church, the common mother of the faithful, the 'pillar and ground of the truth,' revolt from Christ also; but we mean a Church which, from incorruptible seed, begets children for immortality, and, when begotten, nourishes them with spiritual food (that seed and food being the Word of God), and which, by its ministry, preserves entire the truth which God deposited in its bosom. This mark is in no degree doubtful, in no degree fallacious, and it is the mark which God himself impressed upon his Church, that she might be discerned thereby. Do we seem unjust in demanding to see this mark? Wherever it exists not, no face of a Church is seen. If the name, merely, is put forward, we have only to quote the well-known passage of Jeremiah, 'Trust ye not in lying words, saying, the temple of the Lord, the temple of the Lord, the temple of the Lord, are these' (Jer. 7:4). Is this house, which is called by my name, become a den of robbers in your eyes?' (Jer. 7:11). …
Let our opponents, then, in the first instance, draw near to Christ, and then let them convict us of schism, in daring to dissent from them in doctrine. But, since I have made it plain that Christ is banished from their society, and the doctrine of his gospel exterminated, their charge against us simply amounts to this, that we adhere to Christ in preference to them. For what man, pray, will believe that those who refuse to be led away from Christ and his truth, in order to deliver themselves into the power of men, are thereby schismatics, and deserters from the communion of the Church? (On the Necessity of Reforming the Church, as cited by Schaff, ibid., bk. VIII, ch. 13, sec. 98)
D.S. Warner, d. 1895
If the doctrine of Christian union is true, then division and sectarianism are a great and prodigious evil, and therefore ought to be abolished. Who then will lay the axe to the root of this corrupt tree to help cut it down? (unknown)
David Matthew, 2006
Whenever the Lord's Supper came up for united celebration, some of the old barriers proved insurmountable, even if only between the charismatic Catholics and the rest. Either the Supper was tactfully omitted or groups of believes, 'all one in Christ Jesus', made their way from united sessions into separate rooms to 'take holy communion'. (The Covenant Meal. [England: Leaf Publishing] Loc. 795 of 2073 [38%], Kindle version)
Jeremiah Briggs, 2010
The work of staying in step is now all that can be done. You have to listen to the tune and sometimes its not all that easy to hear. But one thing is for sure, if you're listening to the wrong tune you will know because you'll be out of step with all of the other dancers. It won't be hard to tell; you'll be bumping into everyone else in the dance, and always going in the wrong direction. You will always feel out of place and apart from all the other dancers. You'll be alone because you are dancing alone. (Jeremiah Briggs Art & Illustration, April 8, 2010)