Why did Christianity survive at all?

Open persecution: There is considerable evidence that while the Christians did suffer occasional persecution, particularly around 250 and in the Great Persecution of 303-311, Christian tradition seems to have exaggerated the extent of such persecution.

Richard Holland sees the accusations laid against Emperor Nero to be far fetched, saying in Nero: The Man Behind the Myth, page 168, the lack of an authentic and specific early Christian source for a mass persecution under Nero is very odd. It is probably true that Nero blamed the Christians for starting the Great Fire, in order to quash rumours that he himself had been involved, but Holland says that even the words written by Tacitus in Annals 15.44, the only source that mentions a 'great multitude' of victims, can be interpreted as referring to a smaller number of Christians. On page 179, Holland says the Neronian persecution of Christians was an isolated phenomenon, not repeated elsewhere in the Empire at that time.

Origen tells us that up to his own time, few Christians actually died for their faith, and that it was easy to count them:

Contra Celsum, 3.8: For in order to remind others, that by seeing a few engaged in a struggle for their religion, they also might be better fitted to despise death, some, on special occasions, and these individuals who can be easily numbered, have endured death for the sake of Christianity

Edward Gibbon says in The Decline and Fall of the Roman Empire, D.M. Low abridgement, page 200, We might conclude about the attitude of the emperors to the Christians:

• That a considerable time elapsed before they considered the new sectaries as deserving of the attention of government.
• That in the conviction of any of their subjects who were accused of so very singular a crime, they proceeded with caution and reluctance.
• That they were moderate in the use of punishments; and
• That the afflicted church enjoyed many intervals of peace and tranquillity.

Underground churches: It appears that Christian churches were quite openly established in the early years of Christianity.

Michael Grant reports, in The Emperor Constantine, page 190, there was already a house-church of the early second century at St Prisca in Rome, and that similar house-churches are recorded at Rome in the 230s. Grant says quite a number of Christian churches had existed in Rome by the late third century and that they were not by any means all humble house-churches.

Edward Gibbon talks (ibid, page 176) of the existence of opulent churches in Rome, Milan, Carthage and elsewhere, by the close of the third century, many with considerable estates bestowed on them. He says that during the Great Persecution, it was enacted that the churches, in all the provinces of the empire, should be demolished to their foundations. But you can't require their demolition unless churches actually exist.

Why would a religion sprout in such an environment? The survival and success of Christianity through its early years were not assured. Arnold J. Toynbee studied the patterns of civilisation throughout human history and found that successful religions followed a pattern during their establishment phases. Some of the conclusions he reached during his career have been criticised as those of a Christian moralist rather than of a historian, but his work has also been praised as a stimulating answer to the specialising tendency of modern historical research.

Toynbee says in A Study of History (Somervell 2-volume abridgement, volume 1, page 426), that the victory of the Christian Church in the Roman Empire could not have been won if the Fathers of the Church, from St. Paul onwards, had not exerted themselves, during the first four centuries of the Christian era, to translate the Christian doctrine into terms of Hellenic philosophy; to build up the Christian ecclesiastical hierarchy on the pattern of the Roman civil service; to mould the Christian ritual on the model of the Mysteries; and even to convert pagan into Christian festivals and replace pagan cults of heroes by Christian cults of saints.

The responses of Christians under persecution are akin to individual experiences that we often come across in our life, when we pass through troubles and hardships.

In many instances around the world, Christianity has become more deeply rooted in the hearts of its followers under persecution than in prospering and commanding times. Rather Christians tend to move away from their faith in an environment where there is no opposition to their faith or when everything is conducive in their life. Reason may be because, they are busy enjoying the earthly fruits and pleasures when they are in their comfortable phases of life rather than turning to Christ. During their hard times, Christians tend to turns towards Bible and to Christ and try to take an inspiration and example from their savior Himself!

Matthew 27:28-31 (NET)
They stripped him and put a scarlet robe around him, and after braiding a crown of thorns, they put it on his head. They put a staff in his right hand, and kneeling down before him, they mocked him: “Hail, king of the Jews!” They spat on him and took the staff and struck him repeatedly on the head. When they had mocked him, they stripped him of the robe and put his own clothes back on him.

Verses like these in Bible gives immense courage and motivation for Christians who undergo persecution and strengthen their faith further. The application of such verses leads them towards comparison of their life to that of their savior who was God incarnate Himself but lived just like an ordinary man and suffered humiliations.

The comparison may be something like this.

We Christians though just being ordinary humans, will never tolerate in our “real life situations” similar humiliations as suffered by Christ. Any wrong accusation or undue insult against and there would attempts to tell the world that it is not true by shouting from the rooftops. Many times, they will not even accept or bear a single word said against them by someone, even though it may have been warranted in some circumstances due to their behavior. Compare this with above verse from Mathew.

In contrast, Christ though capable of doing wonders (and He did so, to help/heal others and not to show off his powers) to annihilate his detractors and accusers, just surrenders to them meekly. He being the Creator of the very Human being who were insulting Him, Christians acknowledge, how much He tolerated the insults and putdowns from His own creations. With so much of divine power, power even to raise the dead, He could have easily overcome these people humiliating Him. He did not do it because He came for this very purpose. To suffer and die for sinners.

When you acknowledge this behavior of your savior and His suffering for no fault of His, you tend to surrender all the hardships and persecution to Him to take care of them for you. When you do so with a fervent and honest heart and with your mind completely saturated with an urge for having an experience with Him, that is the time you become fully convinced that there is nothing in this world, more appealing than with having a true relationship with your savior and dying for Him. This is the time you become more assured that Jesus being divine did suffer so much then why can’t I, who is just the dust of the earth as compared to the Son of God.

You also realize that Jesus is truly the God incarnate or otherwise he would have behaved like any other human with a human instinct to demonstrate and use His powers to score points against the humanity by behaving like a powerful king or like an entertaining superman.

So it is sheer convincing power of verses in the Bible and the example of Christ that keeps them going under any troubles and persecution.

Short Answer = Faith. Long Answer = History

Why did it Thrive?

From a secular perspective, the history of the church is highly correlated with the success of Western Europe. What follows is drawn highly from A World History of Christianity by Adrian Hastings.

Around the time of Charlemange, for example, Christianity was the primary religion in only a handful of countries in the former Roman Empire, including Ireland, England, France, Italy, and Germany. Christianity was not even officially the religion of Russia, for example, until 988. By contrast, while Christianity had spread across the East through Syria, Iraq, and all the way to China and Mongolia by 600, the spread of Islam (beginning in 622) drastically reduced the power and influence of the church across the East even through the present day. Indeed, when the Turks were invading Southern Europe from the East and the Moors through Spain in the West, one could argue that the Church faced a geographical "crisis," that coincided with "the Dark Ages," being restricted almost entirely to Western Europe. Additionally, the Roman Catholic church was arguably at its nadir in the 1300 & 1400s - a period which corresponded to the aftermath of the Black Death and the isolation of Europe after the heyday of the High Middle Ages in the 1200s and the Crusades in the 1100s.

In contrast, even though the church in Western Europe was rent by schism in the 1500s and 1600s, Christianity was spread to the Americas, reintroduced into East Asia by the Jesuits, and generally experienced marked growth. During this same period, the Italian Renaissance and the Enlightenment which followed were driving factors in the re-emergence of Western European power. While historians would argue over the relative importance of, for example, the printing press (Gutenberg's Bible in 1453 as the classic example), the Age of Exploration (including Henry the Navigator, Columbus, Magellen, etc...), neither the documented rise of Western Europe nor the general period in which it occurred over its neighbors is not.

In no way do I mean to denigrate the fundamentals of Christianity in its success. Additionally, several scholars have argued that Western Europe succeeded because of Christianity, and not the other way around. Max Weber, in The Protestant Work Ethic, argued that the Protestant understanding of Scripture in particular, led to the rise of free markets and wealth. More recently, Rodney Stark "The Victory of Reason", is subtitled "How Christianity Led To Freedom, Capitalism, And Western Success," which gives you an idea of what the book is about. Not without controversy, Stark traces property rights, democracy, and the rule of law through Christianity into the rise of Western Europe, as I alluded to here).

The point is that the "success" of the church is highly correlated with that of Western Europe. It would be difficult, in a scholarly answer, to distinguish the two.

How did it survive long enough to thrive?

The question, however, is not what fundamentals of Christianity caused it to thrive, but what got it to the place where it could survive in the first place. Foxe's Book of Martyrs, The Acts of Perpetua, and the Martyrdom of Polycarp all point to that answer.

They knew God was greater than their present circumstance.

There, people fundamentally believed what Paul said in 1 Corinthians 15:55

"Where, O death, is your victory? Where, O death, is your sting?"

Or, as Paul would state in Romans 8:

35 Who shall separate us from the love of Christ? Shall trouble or hardship or persecution or famine or nakedness or danger or sword? 36 As it is written: “For your sake we face death all day long; we are considered as sheep to be slaughtered.” 37 No, in all these things we are more than conquerors through him who loved us. 38 For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, 39 neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord

People, fully committed to an idea, are powerful. In Genesis 11, God even admits to this, saying:

Nothing will be restrained from them, which they have imagined to do.

People who know that even death is not an impediment are even more of a force to be reckoned with. Christianity survived due to God's grace, to be sure, but also because of faith. Those who came before us had tenacity, yes, but faith. As a result, as Hebrews 11 says:

32 And what more shall I say? I do not have time to tell about Gideon, Barak, Samson and Jephthah, about David and Samuel and the prophets, 33 who through faith conquered kingdoms, administered justice, and gained what was promised; who shut the mouths of lions, 34 quenched the fury of the flames, and escaped the edge of the sword; whose weakness was turned to strength; and who became powerful in battle and routed foreign armies. 35 Women received back their dead, raised to life again. There were others who were tortured, refusing to be released so that they might gain an even better resurrection. 36 Some faced jeers and flogging, and even chains and imprisonment. 37 They were put to death by stoning;[e] they were sawed in two; they were killed by the sword. They went about in sheepskins and goatskins, destitute, persecuted and mistreated— 38 the world was not worthy of them. They wandered in deserts and mountains, living in caves and in holes in the ground.

39 These were all commended for their faith, yet none of them received what had been promised, 40 since God had planned something better for us so that only together with us would they be made perfect.

There are a number of reasons Christianity survived in the face of persecution.

First, Christianity began at a time when the Romans were working to make it easy to travel throughout the Mediterranean, in order to facilitate management of their growing empire. Paul and other early missionaries were therefore able to spread the gospel faster than any religion had ever been spread before. The Roman historian Suetonius reports that Christians were causing a disturbance in Rome during the reign of Claudius, less than two decades after Jesus' crucifixion. So by that time the church had already spread from Jerusalem to Rome and had gathered enough of a following to cause others to notice. (Priscilla and Aquila, mentioned in Acts 18, were probably involved in this disturbance.)

Nero's persecution also contributed to the growth of Christianity in two ways. First, it was localized; Christians outside Rome were generally left alone. Second, Nero was not consistent in explaining his reasons for the persecutions.

After a massive fire swept through Rome, destroying or damaging nearly 3/4 of the city, many Romans suspected Nero of having set the fire himself. To deflect attention, Nero chose a convenient scapegoat. Tacitus explains:

Consequently, to get rid of the report, Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Christians by the populace. Christus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus, and a most mischievous superstition, thus checked for the moment, again broke out not only in Judaea, the first source of the evil, but even in Rome, where all things hideous and shameful from every part of the world find their centre and become popular. Accordingly, an arrest was first made of all who pleaded guilty; then, upon their information, an immense multitude was convicted, not so much of the crime of firing the city, as of hatred against mankind. Mockery of every sort was added to their deaths. Covered with the skins of beasts, they were torn by dogs and perished, or were nailed to crosses, or were doomed to the flames and burnt, to serve as a nightly illumination, when daylight had expired.

So although Nero began rounding up Christians under the pretext of accusing them of arson, he ended up executing them for the vague crime of "hatred against mankind." The Romans, already suspicious of Nero for the great fire, saw these persecutions as evidence of Nero's cruelty.

Though Tacitus was not a fan of Christianity, he notes that a number of Romans converted because of the persecutions.

Nero offered his gardens for the spectacle, and was exhibiting a show in the circus, while he mingled with the people in the dress of a charioteer or stood aloft on a car. Hence, even for criminals who deserved extreme and exemplary punishment, there arose a feeling of compassion; for it was not, as it seemed, for the public good, but to glut one man's cruelty, that they were being destroyed.

Christians would be persecuted at various times and places for nearly three centuries, but later persecutions followed the same pattern as Nero's; persecution was personality-driven, sporadic and localized, or poorly enforced by distant governors. No persecution was so extensive as to threaten the existence of the global church.

The Christian apologist Tertullian, who lived in the late 2nd century, noted the connection between persecution and church growth:

[I]t is quite true that it is our desire to suffer, but it is in the way that the soldier longs for war. No one indeed suffers willingly, since suffering necessarily implies fear and danger. Yet the man who objected to the conflict, both fights with all his strength, and when victorious, he rejoices in the battle, because he reaps from it glory and spoil. It is our battle to be summoned to your tribunals that there, under fear of execution, we may battle for the truth. But the day is won when the object of the struggle is gained. This victory of ours gives us the glory of pleasing God, and the spoil of life eternal. But we are overcome. Yes, when we have obtained our wishes. Therefore we conquer in dying; we go forth victorious at the very time we are subdued. … The oftener we are mown down by you, the more in number we grow; the blood of Christians is seed.