How can we know that the Catholic Church is the one founded by Christ?
We can know that the Catholic church is the one founded by Christ because it is the only one with all the characteristics (marks) that Jesus gave to His Church; that is, He made it one, holy, catholic and apostolic.
We say the Catholic Church is one because all of its members profess the same faith, participate in the same sacraments, and obey the Roman Pontiff—the Vicar of Christ (Ephesians 4:4-5).
Jesus never spoke of a plurality of churches, but of "my church", when He first promised Peter that he would make him the rock foundation of the Church He was about to establish (Matthew 16:18-19). The Church is always pictured in the New Testament as visibly one, presided over by Peter, who represent's Christ, telling all men until the end of time to believe only what Jesus and his apostles taught, to obey His and their commands, to worship as He commanded.
Christ plainly foretold that the gates of hell would never prevail against His Church, and that He would provide for its unity by His own presence and the power of the Holy Spirit. It is granted that the private judgment of the individual naturally brings about disunion in the Church, but Christ ensured its unity by a special supernatural grace, which He asked of His Father the night before He died. His prayer for unity to the Father was:
"That they may all be one; even as thou, father, art in me, and I in thee, that they also may be in us… The glory which thou hast given me I give to them, that they may be one even as we are one, I in them and thou in me, that they may become perfectly one" (John 17:21-23).
St. Paul insists on the unity of the Church in all his epistles. Although he mentions individual local churches in certain cities (dioceses), he teaches clearly that they are parts of the one Church in every place (I Thessalonians 1:8; I Corinthians 1:2; II Corinthians 2:14). The Church is not a mere organization that may be divided and subdivided like a nation or club, but a divine organism with its own inherent principle of life. It is Christ's Mystical Body, of which He is the head and all Christians are members. It is founded by one Lord, given life by one Spirit, entered into by one baptism, ruled by a united body of bishops, and having one aim, the glory of God and the salvation of men's souls (Romans 12:4-8; I Corinthians 12:12-27; Ephesians 4:3-16).
The Catholic Church is holy because Jesus, its Founder, and its Soul, the Holy Spirit, are holy. The Church teaches holy doctrine and gives its members the means of living holy lives, thus producing saints in every age (Ephesians 5:25-27). The founders of other churches—Luther, Calvin, Zwingly, Wesley—were but men, and in no way remarkable for heroic virtue.
The catholic Church is holy, because of her intimate union with Christ as His Bride (Ephesians 5:23-32) and His Mystical Body (I Corinthians 12:27; Ephesians 1:22; 4:12; 5:30). Catholics are a chosen "people" and a "holy nation" because they are branches of the true Vine, Jesus Christ. Although people outside her fold may, through invincible ignorance, be members of the Church in desire, and thus share in her divine life, their churched are "cast forth as a branch and withers" (John 15:6).
The Catholic Church is catholic, or universal, because it is for all peoples of all eras, because all mankind is called by the grace of God to salvation (Mark 16:15). The Catholic Church alone is universal in time, doctrine, and extent. She has existed in perfect continuity from the time of Christ, and she will last until His second coming. She teaches all His gospel, and administers all His divine means of salvation. She is not confined to any particular region or nation, but is widespread among all the nations of the world. Indeed, until the year 1517 there was no other Christian religion besides the Catholic Church! All others are merely imitations of the real thing.
To say the Church is "apostolic" implies that the true Church is the Church which Christ commissioned His apostles to establish under the supremacy of St. Peter. The true Church must trace its origin in unbroken line to Jesus and the twelve apostles. Before giving His divine commission to the apostles, Christ insists on His divine commission from His heavenly Father.
"As the Father has sent me, even so I send you" (John 20:21).
The only Church that can rightly claim that its origin is not due to a break with the past is the Catholic Church. The European Protestants broke with the Apostolic Church at the time of Luther's revolt (1517-1520), and the English Protestants (1559) when King Henry VIII made Parker the first Protestant Archbishop of Canterbury.
When the early Catholics wished to use a most convincing argument to prove the true Church, they always appealed to the fact of its apostolic origin. We find them compiling authentic lists of legitimate bishops, especially with regard to the Apostolic See of Rome. As early as the second century we find the Syrian Hegesippus and the Greek Irenaeus, Bishop of Lyons, maintaining that the source and standard of the faith is the apostolic Tradition, handed down in an unbroken succession of bishops.
"But since it would be very long in such a volume as this to count up the successions in all the churches, we confound all those who in any way, whether through self-pleasing or vainglory, or through blindness or evil opinion, gather together otherwise than they ought, by pointing out the tradition arrived from the Apostles of the greatest, most ancient, and universally known Church, founded and established by the two most glorious Apostles, Peter and Paul, and also the faith declared to men which through the succession of bishops comes down to our times" (Irenaeus, Adversus Haereses, 3:3).