That they may be one, even as we are one ,  Christ cries out to his Father; that they may all be one; even as thou, Father, art in me and I in thee; that they also may be in us .  This exhortation to unity flows in a constant stream from the lips of Jesus, for every kingdom divided against itself is laid waste, and no city or house divided against itself will stand .  It is a teaching which is converted into a vehement desire: And I have other sheep that are not of this fold; I must bring them also, and they will heed my voice. So there shall be one flock, one shepherd . 
What beautiful tones our Lord uses to express this doctrine! He multiplies words and images so that we may understand it, so that this passion for unity may remain engraved on our souls. I am the true vine and my Father is the vinedresser. Every branch of mine that bears no fruit, he takes away, and every branch that does bear fruit he prunes that it may bear more fruit... Abide in me, and I in you. As the branch cannot bear fruit by itself unless it abides in the vine, neither can you, unless you abide in me. I am the vine, you are the branches. He who abides in me, and I in him, he it is that bears much fruit, for apart from me you can do nothing . 
Do you not see how those who separate themselves from the Church, even though they are full of foliage, quickly dry up, and their very fruits are converted into a living bed of worms? Love the holy, roman, apostolic Church. One! For as Saint Cyprian writes: He who reaps elsewhere, outside the Church, dissipates the Church of Christ .  And Saint John Chrysostom insists: Do not separate yourself from the Church Nothing is stronger than the Church. Your hope is the Church; your salvation is the Church; your refuge is the Church. It is higher than the heavens, and broader than the earth; it never grows old, its vigour is eternal . 
To defend the unity of the Church is to live very united to Jesus Christ who is our vine. How? By growing in fidelity to the perennial Magisterium of the Church: For the Holy Spirit was promised to the successors of Peter not that they should manifest a new doctrine by his revelation, but rather that with his assistance, they should religiously safeguard and faithfully teach the revelation that was handed down through the Apostles — the deposit of faith .  By venerating this Mother of ours without stain, and loving the Roman Pontiff, we will preserve unity.
I ask our Lord each day to expand my heart, that he may continue to supernaturalize the love he has put in my soul for all men, without distinction of race, nationality, cultural condition or wealth. I sincerely esteem all men, Catholics or not, those who do believe in something and those who do not. I feel sorry for these unbelievers. But Christ founded only one Church: he has only one Spouse.
The union of all Christians? Yes. Even more: the union of all those who believe in God. But there exists only one true Church. There is no need to rebuild it out of pieces dispersed throughout the world, and it does not need to go through any sort of purification in order to be finally cleansed. The spouse of Christ cannot be adulterous, for she is incorruptible and pure. Only one house knows and safeguards the inviolability of only one bridal bed with chaste modesty. She preserves us for God, she destines for the kingdom the children she has begotten. Anyone who separates himself from the Church unites himself with an adulterer; he leaves behind the promises of the Church and he who abandons the Church of Christ will not achieve the rewards of Christ . 
Now we can understand better how the unity of the Church leads to her holiness, and how one of the principal aspects of her holiness is that unity centred on the mystery of the one and triune God. There is one body and one spirit, just as you were called to the one hope that belongs to your call; one Lord, one faith, one baptism; one God and Father of us all, who is above all and through all and in all . 
Holiness means none other than union with God; a greater intimacy with the Lord, more sanctity. The Church has been willed and founded by Christ, who carries out in this way the will of the Father; the Spouse of the Son is assisted by the Holy Spirit. The Church is the work of the Blessed Trinity; she is holy and our mother, our Holy Mother the Church. We can admire in the Church one perfection which we could call original, and another final, eschatological. Saint Paul refers to both of them in his letter to the Ephesians. Christ loved the church and gave himself up for her, that he might sanctify her, having cleansed her by the washing of water with the word, that he might present the church to himself in splendour, without spot or wrinkle or any such thing, that she might be holy and without blemish . 
The original and constitutive holiness of the Church can be hidden—but never destroyed since it is indefectible: The powers of death shall not prevail against it .  It can be veiled from human eyes, as I was saying, in certain moments of obscurity, which can become almost universal. But Saint Peter applies to Christians the title of gens sancta ,  a holy nation. And being members of a holy nation, all the faithful have received a call to holiness, and they must strive to correspond to grace and to be personally holy. Throughout history and now as well, there have been so many Catholics who have truly sanctified themselves: young and old, single and married, priests and lay people, men and women.
But it happens that the personal sanctity of so many faithful—then and now—is not something externally apparent. Frequently we do not recognise the ordinary people, common and holy, who work and live alongside us. From an earthly viewpoint, what stands out most is sin and unfaithfulness: these are more conspicuous.
Gens sancta , a holy nation, composed of creatures with infirmities. This apparent contradiction marks an aspect of the mystery of the Church. The Church, which is divine, is also human, for it is made up of men, and men have their defects: Omnes homines terra et cinis ,  we men are dust and ashes.
Our Lord Jesus Christ, who founded the holy Church, expects the members of this people to strive continually to acquire sanctity. Not all respond loyally to his call. And in the spouse of Christ, at one and the same time, both the marvel of the way of salvation and the miseries of those who take up that way are visible.
from Saint Josemaria's homily “Loyalty to the Church,” published in the book In Love with the Church .  John 17:11
 John 17:21
 Mt 12:25
 John 10:16
 John 15:1‑5
 St Cyprian, De catholicae Ecclesiae unitate , 6; PL 4, 503
 St John Chrysostom, Homilia de capto Eutropio , 6
 First Vatican Council, Dogmatic Constitution on the Church , Dz‑Sch 3070 (1836)
 Eph 5:25‑27
 Mt 16:18
 1 Pet 2:9
 Sir 17:31