The pope is the visible head of the Catholic Church since he is considered the Vicar of Christ. Being Vicar is somewhat like being an ambassador.
An ambassador is appointed by the king, president, or prime minister of a nation, to represent the head of state in another country. The pope is ambassador or Vicar of Christ in that he represents Jesus to all the believers on earth. He has been given authority from Christ to act in the name and in the person of Christ.
Catholic theology teaches that Jesus had a three-fold mission: prophetic (to teach), priestly (to sanctify), and kingly (to govern, rule, or shepherd). The Catholic Church continues the same prophetic mission of Christ in her teaching authority (called the magisterium, from the Latin word for teacher, magister). She continues the same priestly mission through the Seven Sacraments, and she continues the same kingly mission in her hierarchy (pope, bishops, and pastors).
As Vicar of Christ, the pope possesses full, supreme, immediate, and universal authority to run the Church. This means that he can create or suppress dioceses; appoint or depose bishops; name cardinals; and create, suppress, or interpret church law. He cannot create, change, or eliminate any Divine Law (like the Ten Commandments) or the Natural Moral Law (like the immorality of abortion or euthanasia); however with any human, man-made ecclesiastical (church) laws, he has the last and final word. There is no appeal above the pope’s authority, since he is considered the Vicar of Christ on Earth.
In addition to being the Vicar of Christ, the pope is simultaneously the bishop of Rome. The first pope (the first bishop of Rome) was Saint Peter. He was martyred in Rome under the Emperor Nero around 64 AD. If Saint Peter had remained in Jerusalem for his entire life, then the bishop or patriarch of Jerusalem would be pope today. Peter preached and served the Christian community in Rome. His bodily remains are buried directly under the altar of Saint Peter’s Basilica in Rome, the Church where the pope most often celebrates Mass. After Peter died, his successor, Linus, took over as bishop of Rome. He was followed by Cletus and Clement and so on. Since Saint Peter, there have been 266 popes, with Pope Benedict XVI being the current one (which also makes him the 265th successor of Saint Peter).
Jesus said, “Blessed are you, Simon bar-Jonah. For flesh and blood has not revealed this to you, but my heavenly Father. And so I say to you, you are Peter, and upon this rock I will build my church, and the gates of the netherworld shall not prevail against it. I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven” (Matthew 16:18–19). What we cannot see in the English version is better seen in the Greek since the name “Peter” in Greek is Petros, and the Greek word for “rock” is petra. Peter, therefore, means “rock.”
Catholics interpret Matthew 16 to mean that Saint Peter and his successors (the popes) have full authority to teach (papal infallibility), to govern (papal primacy), and to preside over all acts of sacred liturgy. Each of these successors to Peter acquired these powers the moment he became pope.
When the Roman Empire’s three-hundred-year persecution of Christians was brought to an end with the Edict of Milan in 313 by Constantine, the bishop of Rome became a prominent figure. He and the patriarchs of Jerusalem, Antioch, Alexandria, and Constantinople, were given highest honors among their peers. When the Western empire fell to the Barbarians at the sack of Rome in 476 AD, only the church remained intact. Roman law, government, and culture collapsed, yet it was the bishop of Rome who retained respect, honor, and authority even after the Latin Emperor abdicated. Pope Leo (I) the Great convinced the fierce and ferocious Attila the Hun not to sack Rome in 452, and he did so without any army or weapons whatsoever. This is a clear indication of the respect and authority accorded to the bishop of Rome at that time.
The popes increased in power and prestige over time as more and more Barbarians invaded Europe, settled down, and began to form their own kingdoms and principalities. The patriarch (bishop) of Constantinople, on the other hand, while prominent and important, never got more powerful than his contemporary the Eastern Byzantine Emperor, since the Emperor kept the throne until well into the fifteenth century, when Emperor Constantine was deposed by the Ottoman Muslim Turks in 1453.
When Pope Leo III crowned Charlemagne the first Holy Roman Emperor in 800 AD, it furthered the political prestige and influence of the papacy since many subsequent popes reminded emperors and kings that their temporal (or secular) authority was contingent on the religious authority to anoint those emperors and kings as monarchs. Excommunicated rulers were not owed any allegiance or obedience by their subordinates, hence, many secular rulers in Western Europe had no choice but to respect the position of the pope.
After the Papal States (those areas over which the pope was secular and temporal ruler) were captured and assumed into the new Italian nation under King Victor Emmanuel in 1870, the pope ceased to be a significant temporal power. Even after the Lateran Treaty of 1929, by which the Vatican City was recognized as a sovereign, independent nation (the smallest country in the world), the spiritual authority of the pope never diminished in the Catholic Church.
The First Vatican Council (1869–1870) defined the dogma of papal infallibility, but the Second Vatican Council (1962–1965) also taught that the teaching authority of the pope, even when noninfallible, required religious submission of intellect and will by the faithful (Lumen Gentium #25; CIC #749-750; CCC #891-892).