I concede that even had Christianity not been adopted by the Roman Empire, it might still somehow have survived, but there is no reason to believe so..
You are right. There is no reason. ...Unless God is behind it.
Christianity was thriving even during the Roman persecutions of the first centuries, not just surviving, as you imply.
Also consider:
Christianity is NOT related to the traditional religions of China, has NOT been adopted by its government. European missionary efforts there began mostly with Hudson Taylor, around 1865. There were about 227 million people in China and 581 missionaries by 1887. The rate of Christianity's growth was negligible--perhaps to 1% of the population by 1950, when Communism entered as government.
Quickly, all foreign missionaries were expelled and some were executed. National Christians were pressured to recant or face imprisonment, re-education, and/or death. What's left of the scant church was driven underground, while the Chinese gov't created a parody of Christianity, the Three-Self Church. This Communist-party church borrowed whatever suited its purposes from the Bible, like obeying leaders, etc., and forbade teaching Christ as a resurrected King who is coming again (they just didn't like the thought of competition). The "priests" are appointed by the government, and these priests must act like spies to report any real individual spiritual devotion that develops to the Party.
What is the number of Christians in China, today? Reliable estimates put it at at least 10% of 1 billion people. Ten years ago, for every 50 days that passed, another 1 million Chinese became Christian. (At that rate, in less than 1 year the WT's current membership would be reached) It is still mostly underground, but very active. There are more Christians in China right now than are members of the Chinese Communist party. There might be more Christians there than in the US.
There were no knife-point conversions here, in fact, it has been the opposite. Christians are still regularly arrested and beaten, and occasionally beaten to death (some provinces have tougher cops than others.) People who print or distribute Christian literature (that isn't printed by the communist party--which, by the way, only prints one Christian magazine for all of the registered Chinese Christians to show to any visiting foreigners) are also imprisoned for years. They lose their apartments and jobs. I've met and talked with folks who were tortured there within the last ten years.
Now, if Christianity is always bound to die except for sponsoring governments, it should have stayed within the Western World, and never thrived where people are threatened to remain unconverted at knifepoint. It seems strange to see that many times it grows even better in these conditions.
So, logically, you are right--there is no reason to suspect that it would survive. But history keeps showing that it does not just survive, but illogically thrives.
Why? My thoughts turn back to the Bible.
Daniel 9:35 But the rock that struck the statue became a huge mountain and filled the whole earth.
Psalm 118:22, 23:
`The stone the builders rejected
has become the capstone;
the Lord has done this,
and it is marvelous in our eyes' ?
PS 127:1 Unless the LORD builds the house,
its builders labor in vain.
ISA 43:13 Yes, and from ancient days I am he.
No one can deliver out of my hand.
When I act, who can reverse it?
MATT 16:18 And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it.
Acts 22:38 (Gamaliel speaking) "Therefore, in the present case I advise you: Leave these men alone! Let them go! For if their purpose or activity is of human origin, it will fail. [39] But if it is from God, you will not be able to stop these men; you will only find yourselves fighting against God."
bebu