Unitatis Redintegratio §2

² What has revealed the love of God among us is that the Father has sent into the world His only-begotten Son, so that, being made man, He might by His redemption give new life to the entire human race and unify it. Before offering Himself up as a spotless victim upon the altar, Christ prayed to His Father for all who believe in Him: "that they all may be one; even as thou, Father, art in me, and I in thee, that they also may be one in us, so that the world may believe that thou has sent me". In His Church He instituted the wonderful sacrament of the Eucharist by which the unity of His Church is both signified and made a reality. He gave His followers a new commandment to love one another, and promised the Spirit, their Advocate, who, as Lord and life-giver, should remain with them forever. After being lifted up on the cross and glorified, the Lord Jesus poured forth His Spirit as He had promised, and through the Spirit He has called and gathered together the people of the New Covenant, who are the Church, into a unity of faith, hope and charity, as the Apostle teaches us: "There is one body and one Spirit, just as you were called to the one hope of your calling; one Lord, one faith, one Baptism". For "all you who have been baptized into Christ have put on Christ ... for you are all one in Christ Jesus". It is the Holy Spirit, dwelling in those who believe and pervading and ruling over the Church as a whole, who brings about that wonderful communion of the faithful. He brings them into intimate union with Christ, so that He is the principle of the Church's unity. The distribution of graces and offices is His work too, enriching the Church of Jesus Christ with different functions "in order to equip the saints for the work of service, so as to build up the body of Christ". In order to establish this His holy Church everywhere in the world till the end of time, Christ entrusted to the College of the Twelve the task of teaching, ruling and sanctifying. Among their number He selected Peter, and after his confession of faith determined that on him He would build His Church. Also to Peter He promised the keys of the kingdom of heaven, and after His profession of love, entrusted all His sheep to him to be confirmed in faith and shepherded in perfect unity. Christ Jesus Himself was forever to remain the chief cornerstone and shepherd of our souls. Jesus Christ, then, willed that the apostles and their successors - the bishops with Peter's successor at their head - should preach the Gospel faithfully, administer the sacraments, and rule the Church in love. It is thus, under the action of the Holy Spirit, that Christ wills His people to increase, and He perfects His people's fellowship in unity: in their confessing the one faith, celebrating divine worship in common, and keeping the fraternal harmony of the family of God. The Church, then, is God's only flock; it is like a standard lifted high for the nations to see it: for it serves all mankind through the Gospel of peace as it makes its pilgrim way in hope toward the goal of the fatherland above. This is the sacred mystery of the unity of the Church, in Christ and through Christ, the Holy Spirit energizing its various functions. It is a mystery that finds its highest exemplar and source in the unity of the Persons of the Trinity: the Father and the Son in the Holy Spirit, one God.