Chi Rho (early Christian symbol consisting of first two letters of Christ in Greek), Catacomb of Saint Callistus, Rome, c. 250 A.D. Note the addition of the Alpha and Omega (cf. Rev. 1:8).
From ancient times, the Church of Christ has been identified by four distinguishing marks or signs: that she is "one, holy, catholic, and apostolic." These four marks are famously included in the Nicene Creed, one of the definitive statements of early Christianity.
The Church is One. The first mark of the Church refers to her unity, which is safeguarded by the Holy Spirit, and follows the prayer of Jesus from the Last Supper: "May all be one; even as thou, Father, art in me, and I in thee, that they also may be in us, so that the world may believe that thou hast sent me" (John 17:21). Sadly, as though in opposition to Christ's prayer, Christianity today faces ongoing rampant division along doctrinal lines. Oxford University's World Christian Encyclopedia (2001), for example, catalogs more than 33,000 distinct Christian denominations worldwide; and the number has continued to escalate since that time. The Catholic Church, by contrast, has remained united in her doctrine since the first century A.D.
The Apostolic Church placed a great emphasis on the doctrinal unity of her members. Saint Paul, for instance, wrote to believers in Corinth:
I appeal to you, brethren, by the name of our Lord Jesus Christ, that all of you agree and that there be no dissensions among you, but that you be united in the same mind and the same judgment (1 Cor. 1:10).
Christianity's historical writings, or the writings of the Early Church Fathers as they are known, likewise testify to the importance of unity of belief. In them we find Catholic teaching consistently professed on subjects ranging from the Blessed Trinity, salvation and the sacraments, to the papacy, purgatory and the saints. This doctrinal unity, which the Church has maintained from that time forward, is signified by the Pope or Bishop of Rome, who, as the successor to the Apostle Peter, maintains the oneness of the faithful through the power of the Holy Spirit.
The earliest list of the Bishops of Rome that we have comes from Saint Irenaeus, the Bishop of Lyons, whose teacher, Saint Polycarp, was a disciple of the Apostle John. Writing in about the year 185, Irenaeus, having listed the Popes from Peter to Saint Eleutherus, the Pope of his day, goes on to say of the Church of Rome:
But since it would be too long to enumerate in such a volume as this the successions of all the Churches, we shall confound all those who, in whatever manner, whether through self-satisfaction or vainglory, or through blindness and wicked opinion, assemble other than where it is proper, by pointing out here the successions of the bishops of the greatest and most ancient Church known to all, founded and organized at Rome by the two most glorious Apostles, Peter and Paul, that Church which has the tradition and the faith which comes down to us after having been announced to men by the Apostles. For with this Church, because of its superior origin, all Churches must agree, that is, all the faithful in the whole world; and it is in her that the faithful everywhere have maintained the Apostolic tradition (Against Heresies 3:3:2).
The Church is Holy. The second mark of the Church is often misconstrued to mean the Church's individual members are holy. While many in fact are, the second mark points primarily to the fact the Church, having been established by Christ, has been entrusted with Baptism and the Lord's teachings — the means, in other words, to make men and women holy. "All authority in heaven and on earth has been given to me," said Jesus to the Apostles.
"Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age" (Matt. 28:18-20).
Anti-Catholics often attempt to refute the Church's holiness by pointing out the sinful behavior of some of her members. Nonetheless, Jesus Himself described the Church as a field that would contain both wheat and weeds, and as a net that would catch both good and bad fish (cf. Matt. 13:24-30, 47-48). Unfortunately it is true that along with the good people in the Catholic Church there are some wicked as well — among the clergy and laity alike. The sinfulness of the Church's members, though, does not disprove her divine origin any more than the sinfulness of the Israelites — including even some of their leaders, such as King David (cf. 2 Sam. 11:2ff.) — disproved Israel's divine origin.
This having been said, it is also true that the accusations leveled against the Church tend to be grossly exaggerated. Many imply that the majority of Popes were great sinners, for example, when in reality the "bad Popes" amount only to about two dozen of the total 266! This adds up to about 1 in 12, the same ratio of good-to-bad in the Twelve Apostles, handpicked by Jesus Himself. The Church's critics often seem willing to overlook the great good she has brought to mankind over the millennia. Few realize, for instance, that the hospital and university are Catholic inventions.
The Church Catholic. The term "catholic," from the Greek katholikos, means "universal." It refers to the fact that the one faith which Christ brought to earth has, by His command to the Apostles (Matt. 28:18), been spread throughout the earth. This is so because Christ, desiring "all men to be saved" (1 Tim. 2:4), willed that His Church, the instrument of His salvation, be propagated everywhere and throughout all time. We hear this in Christ's promise to Saint Peter, "And I tell you, you are Peter, and on this rock I will build my church, and the powers of death shall not prevail against it" (Matt. 16:18); as well as in the words of Saint Paul, "To [Christ] be glory in the church and in Christ Jesus to all generations, for ever and ever. Amen" (Eph. 3:21).
The first writer on record to use "catholic" as a proper name for the Church is Saint Ignatius, the Bishop of Antioch and a student of the Apostle John, who in about 107 A.D. writes:
Wherever the bishop appears, let the people be there; just as wherever Jesus Christ is, there is the Catholic Church (Letter to the Smyrnaeans 8:2).
Interestingly enough, Antioch, Ignatius' bishopric, happens also to be the place where the followers of Christ were first called "Christians" (Act 11:26). The first written usage of "Trinity" comes from there as well, appearing in a letter of another of her bishops, Saint Theophilus, in about 181. Speaking of the Church's catholicity or universality at about the same time, Saint Irenaeus of Lyon writes:
For the Church, although dispersed throughout the whole world even to the ends of the earth, has received from the Apostles and from their disciples the faith in one God, the Father Almighty, the Creator of heaven and earth … and in one Jesus Christ, the Son of God, who became flesh for our salvation; and in the Holy Spirit, who announced through the prophets the dispensations and the comings, and the birth from a Virgin (Against Heresies 1:10:1).
The Church is Apostolic. The authority which Jesus gave to the Twelve Apostles is readily evident in Scripture. They are commissioned by Him to baptize and teach in His name (cf. Matt. 28:18-20), to preside at the Eucharist in His memory (cf. Luke 22:19), and to forgive sins in His name (cf. John 20:23). Saint Paul describes the Church as being "built upon the foundation of the apostles" (Eph. 2:20).
Of all of today's multiple and diverse Christian assemblies, only the Catholic Church is able to substantiate her claims of authenticity through apostolic succession, the constant line of bishops that has faithfully carried the Apostles' teachings from the first century down to the present day. Referring to this apostolic succession, Paul writes to Saint Timothy, whom he consecrated a bishop:
You then, my son, be strong in the grace that is in Christ Jesus, and what you have heard from me before many witnesses entrust to faithful men who will be able to teach others also (2 Tim. 2:1-2).
Apostolic succession is affirmed by the Church's historical record. As previously mentioned, the Early Church Fathers maintained lists tracing their local bishop back to the Apostles; and used these lists to verify the authenticity of their doctrine and denounce heresy. "If there be any [heresies] bold enough to plant themselves in the midst of the apostolic age," writes Tertullian around 200,
so that they might seem to have been handed down by the Apostles because they were from the time of the Apostles, we can say to them: let them show the origins of their Churches, let them unroll the order of their bishops, running down in succession from the beginning, so that their first bishop shall have for author and predecessor some one of the Apostles or of the apostolic men who continued steadfast with the Apostles (The Demurrer Against the Heretics 32:1).