According to Church tradition, it was under Nero's persecution that the saints Peter and Paul became martyrs in Rome. Several of the New Testament writings mention persecutions and very difficult times. For 250 years there were times when Christians suffered from persecutions, because they refused to worship the Roman emperor. That was seen as treason and punished by execution. But the Christian religion continued to spread throughout the Mediterranean region. In the late fourth century they became the dominant religious force of the Roman Empire.
The first to legalize Christianity was the Armenian king Trdat the Third, who announced it the official religion in Armenia in the year 301. Galerius issued an edict permitting the practice of the Christian religion in April of 311. In 313 Constantine I and Licinius announced toleration of Christianity in the Edict of Milan. Constantine became the first Christian emperor; he had learnt about Christianity from his mother, Helena.
By 391, under the reign of Theodosius I, Christianity had become the state religion of Rome.
When Christianity was legalized the Church used the same provinces for administration as the imperial government and called them dioceses. The Bishop of Rome claimed to be the highest amongst all others and chose the title pope.
During this era, there were several Ecumenical Councils. These were mostly concerned with Christological disputes. The First Council of Nicaea condemned Arianism and produced the Nicene Creed to define the faith. The Council of Ephesus condemned Nestorianism and affirmed the Blessed Virgin Mary to be Theotokos ("God-bearer" or "Mother of God"). Perhaps the most important was the Council of Chalcedon that affirmed that Christ had two natures, fully God and fully man, at the same time. That meant that Monophysitism was condemned.
The Church in the Early Middle Ages saw a "transformation of the Roman world" rather than a "fall of the Roman Empire". With the Muslim invasions of the seventh century, the Western (Latin) and Eastern (Greek) areas of Christianity began to take on distinctive shapes, and the Bishops of Rome were more interested in barbarian kings than in the Byzantine Emperors. This led to the coronation of Charlemagne as "Emperor of the Romans" by Pope Leo III in Rome on Christmas Day, 800.
The city of Rome was affected very much by warfare in Italy during the Early Middle Ages. Emperor Justinian I conquered the Italian kingdom of the Ostrogoths. He made Ravenna, Italy a territory with its own governor, but imperial influence was often limited. After the invasion of the Lombards, Rome had to take care of itself. Thus the popes, out of necessity, found themselves feeding the city with grain from papal estates, negotiating treaties, paying protection money to Lombard warlords, and, failing that, hiring soldiers to defend the city. Eventually, the failure of the Empire to send aid resulted in the popes turning for support from other sources, most especially the Franks.
The High Middle Ages is the period from the coronation of Charlemagne in 800 to the close of the fifteenth century, which saw the fall of Constantinople (1453), the end of the Hundred Years' War (1453), the discovery of the New World (1492), and thereafter the Protestant Reformation (1515).
The "Investiture Controversy", also known as the "lay investiture controversy", was the most important conflict between secular and religious powers in medieval Europe. It began as a dispute in the 11th century between the Holy Roman Emperor Henry IV and Pope Gregory VII. The question was who would control appointments of bishops (investiture). The end of lay investiture meant an important loss of royal power, and a loss for ambitious noblemen, for the benefit of Church reform as the pope intended.
Bishops collected revenues from estates attached to their bishopric. Noblemen who held lands (fiefdoms) hereditarily passed those lands on within their family. However, a king had more control of lands under the domain of his bishops. Kings would give bishoprics to powerful friends. If a king left a bishopric vacant, he got the money until a bishop was appointed, when he was supposed to repay the earnings. This seldom happened. The Church wanted to end this lay investiture because of the vacant sees simony and other problems. Thus, the Investiture Contest was part of the Church's attempt to reform the episcopate and get better bishops.
Pope Gregory VII issued the "Dictatus Papae", which said only the pope may make, break, or move bishops to other sees. The Emperor rejected this. With excommunication and a revolt by his dukes, Henry apologized and was forgiven, though the conflict continued. A similar controversy happened in England between King Henry I and St. Anselm, Archbishop of Canterbury. The English dispute was resolved by a deal in 1107, where the king gave up making bishops but did require an oath of fealty. The later Concordat of Worms ("Pactum Calixtinum") resolved the Imperial investiture controversy with a similar compromise.
The Crusades were a military conflicts conducted by Christian knights for the defense of Christians and for the expansion of Christian domains. Generally, the crusades refer to the campaigns in the Holy Land against Muslim forces sponsored by the Papacy. There were other crusades against Islamic forces in southern Spain, southern Italy, and Sicily, as well as the campaigns of Teutonic knights against pagan strongholds in Eastern Europe, and (to a much lesser extent) crusades against Catharism or other Christian heresies.
The Holy Land had been part of the Roman Empire, and thus Byzantine Empire, until the Islamic conquests of the seventh and eighth centuries. Thereafter, Christians had generally been permitted to visit the sacred places in the Holy Land until 1071, when the Seljuk Turks closed Christian pilgrimages and assailed the Byzantines, defeating them at the Battle of Manzikert. Emperor Alexius I asked for aid from Pope Urban II (1088-1099) for help against Islamic aggression. Instead of sending money, Urban II called upon the knights of Christendom in a speech made at the Council of Clermont on 27 November 1095, combining the idea of pilgrimage to the Holy Land with that of waging a holy war against infidels.
The East-West Schism, or Great Schism, separated the Church into Western (Latin) and Eastern (Greek) branches, i.e. Western Catholicism and Eastern Orthodoxy. It was the first major division since certain groups in the East rejected the decrees of the Council of Chalcedon (see Oriental Orthodoxy), and was far more significant. Though normally dated to 1054, the East-West Schism was the result of an long time of disagreement between Latin and Greek Christendom over papal primacy and certain doctrinal matters like the "filioque". Bad feelings were intensified by cultural and linguistic differences.
The schism became "official" in 1054 when the Pope's legates notified Patriarch Michael Cerularius of Constantinople that he had been excommunicated. A few days later, he excommunicated the legates. Attempts at reconciliation were made in 1274 in Lyon and in 1439 in Basel, but in each case the eastern hierarchs who agreed to the unions were repudiated by the Orthodox as a whole. However, reconciliation was achieved between the West and what are now called the "Eastern Rite Catholic Churches". More recently, in 1965 the mutual excommunications were rescinded by the Pope and the Patriarch of Constantinople, though schism remains.
Both groups are descended from the Early Church, both acknowledge the apostolic succession of each other's bishops, and the validity of each other's sacraments. Though both acknowledge the primacy of the Bishop of Rome, Eastern Orthodoxy understands this as a primacy of honor with limited or no ecclesiastical authority in other dioceses.