From the beginning, the Church met with many challenges to its beliefs, both from within and without. First, the fledgling Church had to avoid being swallowed up by some of the influential religions of the day. Then it had to face major divisions within its own ranks.

The most significant of these internal challenges came from the Gnostics. The name is an umbrella term for a number of groups, each of which thought that it had obtained the secret key to religion. In each case, this hidden knowledge (Greek gnosis, hence the name) centered on the belief that the material world is evil but that the souls of the elect few could struggle to escape it, as well as various esoteric recipes for so doing. Beyond that, each group believed that it alone held the truth, and despised all other Gnostics and all other religions, which were but shadows of the truth.

Gnostic beliefs possibly entered the Church in the time of the Apostles, and Paul may have been opposing such teaching in his letters to the churches at Colossae and Corinth. From the late first century to the middle of the second, orthodox leaders repeatedly clashed with about a dozen different Gnostic groups, each of which tried to convert the Church to its own secret religion.

All Gnostics rejected the Incarnation, owing to their belief that matter is evil. The leaders of two Gnostic groups, Basilides in Egypt and Valentinus in Rome, taught, as the “true” Christian teaching, that Jesus had only appeared to be a man but had in fact been a spirit all the time. This heresy came to be known as Docetism, from the Greek dokesis, “appearance.”

The learned Greek convert Marcion, who came from Asia Minor and was prominent in the church at Rome by 137, combined some Gnostic beliefs, without the emphasis on secrecy, with his own radical biblical criticism. Marcion sought to make Christianity more acceptable to Greek thought by rationalizing it and cutting it loose from its Jewish heritage. In Marcion‟s judgment Paul was the only true Christian among the biblical authors, and he deleted the rest of the biblical text from his own personal canon. Having thus disposed of all the elements of Christianity he considered problematical, and having spelt out a system of beliefs that was largely Gnostic, except that it was not secret, Marcion was surprised to find himself thrown out of the Roman church in 144. He retreated to the Middle East, where he founded a large and successful sect.

Persuasive though they were, Marcion and the Gnostics were divided and offered salvation only to an elite few. The Church was universal and united; the gospel offered forgiveness to all. By the end of the second century, Gnosticism was a spent force and the Church was stronger than ever.

In the 170s, in Asia Minor, an inspiring Christian leader called Montanus claimed to have a major new revelation. He and his two female lieutenants began a thriving cult that was to remain a force in Asia Minor for a century. Its cardinal features were: the claim to have a new revelation, adding to the biblical one; Inspiring, charismatic leadership; the very high standards of behavior demanded of members, much more stringent than the rest of the Church; and the claim to be the only true Christians, so that to reject them was to reject God (all these features were to recur in many other heretical groups). The content of their “New Prophecy” seems strange today: they believed that the kingdom of heaven would shortly descend to earth in their home province of Phrygia in Asia Minor. The purity of their lives, however, and the strength of their Christian witness was such that they won a large following, including the fiery African apologist Tertullian who, fed up with the laxity of his orthodox brethren, ended his life a Montanist.

There were very many other cults and sects at the edge of the Church, such as the Ebionites, Jewish Christians who refused to abandon Jewish customs and rites. The number of such groups increased with time, as those defeated in theological debates frequently formed their own sects. In the 390s, Filastrius, bishop of Brescia, listed 156 distinct heresies, all still flourishing. The orthodox view on such groups was summed up by Cyprian in the 250s: “He cannot have God for his Father who does not have the Church for his Mother.” Augustine was even more succinct: “There is no salvation outside the Church.” This view explains the ferocity with which heresies were fought: they were robbing people of eternal salvation.

Controversies with heretics did, however, have a positive effect in encouraging the Church to clarify its beliefs. Against Marcion and the Gnostics, for example, the Church reaffirmed its faith in the Old Testament, and began to define what was and was not in the New Testament canon. Against Montanus, Church leaders declared the priority of the biblical revelation over subsequent private revelations. In the years after the crucifixion of Jesus, many of his followers were faced with the prospect of suffering a violent death, as had their Lord. Dire rumours circulated about them. The authorities mistrusted them. They had no legal right to exist.

The Roman Empire normally tolerated religions, if their adherents were willing to sacrifice to the emperor. It was a loyalty test. The Jews refused but, being long established, were still tolerated so long as they remained loyal. As Celsus put it, “The religion of the Jews may be highly peculiar, but it is at least the custom of their fathers.” The Christians could offer no such defense.

As long as Christianity remained illegal, Christians were at the mercy of imperial disfavor and popular enmity. Fortunately, no emperor regarded them as enough of a threat to institute a systematic,
empire-wide campaign against them until 249. There were, however, a number of local, temporary persecutions and many martyrs – the word comes from the Greek, “to bear witness.” Misunderstandings clouded the image of Christianity in the popular mind. Eucharistic teaching about feeding from “the body and blood of the Lord” was misinterpreted as cannibalism, and references to communion as a “love feast” gave rise to rumors of incest, orgies, and infant sacrifice. The Roman historian Tacitus, writing in c.115, describes Christians as “a class of men loathed for their vices” and says that after the fire of Rome in 64 they were “convicted, not so much for the crime of arson, as for hatred of mankind.”

These attitudes endured. Writing in c.170, Celsus claimed, “There is a new race of men born yesterday, with neither homeland nor traditions, allied against all religious and civil institutions, pursued by justice, universally notorious for their infamy, but glorying in common execration: these are Christians.”

Popular anger led to many persecutions. At the time that Tacitus was writing, Pliny the Younger, governor of Bithynia in northern Asia Minor, was asking the emperor Trajan, by letter, for advice on dealing with the increasing number of Christians who refused to worship an image of the emperor. He explained that he had been confronted with complaints against the Christians, because of whom the temples were “almost deserted.” This left Pliny with a problem: what to do with Christians who were denounced to him as unfaithful subjects of the emperor?

He wrote to Trajan: “Meanwhile this is the course I have taken with those who were accused before me as Christians. I asked them whether they were Christians, and, if they confessed, I asked them a second or third time with threats of punishment. If they kept to it, I ordered their execution; as for Roman citizens, I noted these down to be sent to Rome.” The emperor replied, approving of Pliny‟s tactics, while urging that the Christians be not hunted, but punished only when denounced by informers. Other emperors were less kind: Nero, Domitian, and Marcus Aurelius all persecuted the church in Rome.

Things did not improve for the Christians. In 177 up to 48 Christians were killed at Lyons in Gaul, after rumors of immorality. Three years later, 12 were martyred at Scilli in North Africa. In 202 the emperor Septimius Severus, worried at the growth of the Church, prohibited conversion to the Christian faith, leading to a major persecution.

Respite came during the reign of Alexander Severus (r.222–35), whose mother was sympathetic to the Christians. The emperor may himself have had a statue of Christ in his home, along with images of Abraham and the deified emperors. The emperor Philip of Arabia (r.244–49) was also a sympathizer and maintained an active correspondence with noted Christian writers. His death in 249 was the signal for vicious persecutions throughout the empire.