• Speaking of His Church, the Saviour called it a kingdom, the kingdom of heaven, the kingdom of God (Matthew 13:24, 31, 33; Luke 13:18; John 18:36);
• He compared it to a city the keys of which were entrusted to the Apostles (Matthew 5:14; 16:19),
• to a sheepfold to which all His sheep must come and be united under one shepherd (John 10:7-17);
• to a vine and its branches,
• to a house built upon a rock against which not even the powers of hell should ever prevail (Matthew 16:18).
• Moreover, the Saviour, just before He suffered, prayed for His disciples, for those who were afterwards to believe in Him — for His Church — that they might be and remain one as He and the Father are one (John 17:20-23); and
• He had already warned them that "every kingdom divided against itself shall be made desolate: and every city or house divided against itself shall not stand" (Matthew 12:25).
• Schism and disunion he brands as crimes to be classed with murder and debauchery, and declares that those guilty of "dissensions" and "sects" shall not obtain the kingdom of God (Galatians 5:20-21).
• Hearing of the schisms among the Corinthians, he asked impatiently: "Is Christ divided? Was Paul then crucified for you? or were you baptized in the name of Paul?" (1 Corinthians 1:13).
• And in the same Epistle he describes the Church as one body with many members distinct among themselves, but one with Christ their head: "For in one Spirit we are all baptized into one body, whether Jews or Gentiles, whether bond or free" (1 Corinthians 12:13).
• To show the intimate union of the members of the Church with the one God, he asks: "The chalice of benediction, which we bless, is it not the communion of the blood of Christ? And the bread, which we break, is it not the partaking of the body of the Lord? For we, being many, are one bread, one body, all that partake of one bread" (1 Corinthians 10:16-17).
• Again in his Epistle to the Ephesians he teaches the same doctrine, and exhorts them to be "careful to keep the unity of the Spirit in the bond of peace", and he reminds them that there is but "one body and one spirit-one Lord, one faith, one baptism, one God and Father of all" (Ephesians 4:3-6).
• Already, in one of his very first Epistles, he had warned the faithful of Galatia that if anybody, even an angel from heaven, should preach unto them any other Gospel than that which he had preached, "let him be anathema" (Galatians 1:8).
Bullet Source:
CATHOLIC ENCYCLOPEDIA: Unity (As a Mark of the Church)