Why did Christianity survive at all? I was reading about ... hold on, what was I reading about? Nevermind, I was URL surfing. Jumping around from one link to another I somehow got to this -


  He [Nero] was known for having captured Christians burned in his garden at
  night for a source of light. (Source)


I rubbed my eyes and squinted at the monitor. I wasn't hallucinating. I would never have believed such an outrageous statement had I not read it on wikipedia. :P

And it gets progressively worse -


  In the words of Tacitus, Christians showed "hatred of the human race"
  (odium generis humani). Christians were thought to use black magic in
  pursuit of revolutionary aims, and to practice incest and cannibalism.
  
  "The Christians are to blame for every public disaster and every
  misfortune that befalls the people. If the Tiber rises to the walls,
  if the Nile fails to rise and flood the fields, if the sky withholds
  its rain, if there is earthquake or famine or plague, straightway the
  cry arises: 'The Christians to the lions!'"


I tried to visualize and think just why would a belief system grow in such an environment. It seems counter intuitive. Let's look at the points -


Open persecution. In modern times most of the countries have laws against religious violence. Persecutions are done by violent mobs but aren't state sponsored (exceptions apply). But with open support of government it would've been much worse.
Small population. Hundreds of Christians are martyred nowadays. But since the population is already so large it doesn't have much effect.
No written Bibles. Imagine hearing sermons from a traveling missionary and then hanging onto that for the rest of your life. Repeated readings of the Bible makes it easier to learn your religion and not forget basic tenets of your faith.
Underground churches. No fellowship and support of other people to let you hang on when times are tough.
Hard to earn your bread. Economic system was organized in a way to favor certain people. In modern times corporate culture has
removed religion and other personal factors from economy. Christians
would've found themselves at the bottom of this socioeconomic system.
And whenever you did catch a fish or log some timber, your house is
open to robbery. Law enforcement doesn't give a rat.


  The Persecution in Lyon was preceded by mob violence, including
  assaults, robberies and stonings.
  Cultural isolation. Marriages, funerals and family life in general were difficult. You can imagine the difficulty by hearing the complaints of minorities in modern society and multiplying that many times over.
  Impracticality. There's little incentive to listen to that Babbling missionary when you've just returned from a Saturday night carnage show at the neighborhood Colosseum. You surely wouldn't want yourself to be cast in the next show, do you?
  
  It astounds my mind; just why would a religion sprout in such an environment? 


Note: Please don't say that it was God's will. And please no quoting Acts 5:39.
The clear and correct answer is simply: God's will. God ensured that it happened.
Now, suppose one rephrased the question as: how could one explain the survival of Christianity in purely athiest terms -- with no mention of God whatsoever -- what is the closest approximation one could do?
Measuring love in units of sacrifice.
There's something fundamentally inspiring about someone willing to die for their beliefs. The following two quotes:

When Christ calls a man, he bids him come and die.
He is no fool who gives up what he can not keep in exchange for what he cannot lose.

has inspired me infinitely more than all the Prosperity &quot;Gospel's&quot;:

Want health / wealth / happiness in this world and heaven in the next? Follow me in praying this little prayer.

The willingness of the Christians to die for their beliefs had the following effects:

it inspired their fellow Christians. If Bob and Alex who I talked to last month decided to be lion's fodder rather than reject Christ -- how can I reject Christ?

it drew the curiosity of not-yet-converted. Their sacrifice / martyrdom could not be explained by the selfish desires of humankind. Thus, their actions must have come from some other source -- and they must have really really believed it, in order to sacrifice their life for it. The more Christians that died -- the more gruesome their death -- the more it drew the curiosity of others to wonder &quot;What did these Christians have that I do not have -- that gave them such a purpose in their life?&quot;


No Written Bibles
We're spoiled in the modern age. I have at least 5 physical copies of the Bible in my room, and quite a few more digital copies. I've only read one copy cover to cover. And when I do read it, I feel it's more of an obligation than an intense desire. Those without the Bible probably had a better understanding of the Bible than I do -- because the verses were precious to them -- they memorized it -- they pondered it -- rather than read it for 15 minutes in a morning quiet time, and forget about it through out the day.
Underground Churches
In personal experiences, I've found it much easier to perceive theological truths during times of solitude + depression ... than times of social happiness. I think the reason is as follows: almost everything in the world exists for one purpose -- to keep us so busy and occupied from birth to death -- that we lack the time to ponder on God.
Because the early Christians had nothing (will I get into university XYZ, who much will company XYZ pay me / year ...), they had no distractions, and this allowed them to a higher understanding of God.
Cultural Isolation / Complaints
I don't think the early Christians complained. Why would they? A perfect Christ died on the cross -- they were instructed to pick up their cross and follow him -- they likely viewed persecution as an honor (to be considered worthy enough) and death to be glorious (yay Heaven!).
Overall Analysis
I think the fundamental problem is that the question looks at &quot;Biblical Christianity&quot; with a modern view of Christianity (i.e. that somehow this life should somehow be enjoyable).
In this sense, I believe that Biblical Christianity -- because it was so heavily persecuted, is a type of Christianity where &quot;conversion = I'm literally ready to be tortured + die for Christ ... because that's what happens to most of the people who convert&quot;
Modern Christianity, is much like &quot;Yeah, these Christians people are pretty nice; they don't drink or do drugs; they hang out every Saturday to sing songs; I'd like to hang out to them.&quot;
And because of this, it's hard to us to understand how the Biblical Christians achieved so much ... much like how we struggle to understand how the 300 at thermopylae did what they did.
In summary:
It was really God's work. But if we must attribute it to help qualities, it's because the early Christians actually believed in Christ and were willing to die for Christ, rather than be luke warm christians.