Has Christianity received a much longer life than its inventers expected?

Yes, it certainly has: Christianity is a mistake. 

The Bible was written by Jews, for Jews, who looked 

to break away from the centuries-old traditions of their elders. 

Christ-based faith was never a final intention of The Bible, 

but creating division, and a chasm within the Jewish Faith, was. 

Christianity will always be the biggest misunderstanding 

that has ever taken place in the history of humanity.

Inventors???? Jesus and God created Christianity. 

Not even close. Jesus Christ, Almighty God in the flesh, Who founded the original and true Christian Church, promised that He would be with it "until the end of time". So it still has a long way to Go.

Probably.

Of course they could have never predicted that it would get co-opted by the Roman Empire, spread at the tip of a sword through various crusades and inquisitions, and propagated by 'missionaries' accompanying conquistadors and colonizers.

No, and since it's creator is God, and He told us that the Gates of Hell would not prevail against His church, I expect Christianity will last until the Earth is finally destroyed and Christians find themselves in heaven. 

Since Christianity was not "invented" by humans, you have no question.

I feel really sad that you ask this Michael. With this attitude you will (unless you change) suffer greatly when Christ returns to take His own to be with Him in paradise.  YOU will be one of those LEFT BEHIND.

Probably.  I doubt they thought it would take thousands of years for their saviour to come back.

Then again, maybe not.  The religion is still around today, and its followers are still saying "just you wait, any day now" as if the crucifixion happened a few weeks ago, so you've got to give them credit for a good marketing plan.

The inventors, did they disappear, were they your brothers?

The authors of the holy bible were the Prophets and faithful of God!

Watch out, otherwise you are going to hell!

You're welcome!

Sp:  inventors.

Religions are not usually invented by one person, although Joseph Smith and L Ron Hubbard did invent their own religions.  But yes, early on the expectation seemed to be that the world would end within the lifetime of some of Jesus's contemporaries.