How did Christianity start?
The death of Christ.
Jesus didn't really start a certain religion, he taught about God and performed miracles, died, came back and went up to heaven alive. Later someone started Catholicism (probably for money) then other religions like evangelical, Lutheran, Orthodox etc. Branched off from there.
When someone found out they could make money talking about jesus and god and whole bunch of other charectors
Here is a good start in your quest for knowledge -
https://en.wikipedia.org/wiki/History_of_Christianity#Early_Christianity_.28c.31.2F33.E2.80.93325.29
Money and power.
When people started to accept Jesus Christ as there Lord and Savior.
Christianity developed from Judaism and although Jesus is usually considered to be the founder, Christianity probably owes its "religion" status to Saul of Tarsus. He was a clever guy and may have been the first century incarnation of Joseph Smith or L. Ron Hubbard.
From Didge's Gospel we learn what really happened.
In Nazareth a pretty lady named Meg was betrothed to Joe Carpenter. She found herself "with child" and rather than admit she'd had it away with Mordechai Butcher she told Joe that it was God's child. No big deal; it's happened a zillion times and Joe believed her.
She gave birth to a brighter-than-average child who was taught that he was God's son. He had no reason to disbelieve his parents and grew up thinking it was true. This led him to a personal position that in most people would have been megalomania but in Little Joe (as his dad liked to call him) was just an acceptance of his divinity.
So he roamed the land sharing his thoughts with anybody who would listen, and his reputation grew.
Of course, ultimately he ran afoul of the authorities and they had him executed but, since you can't kill God, his followers began to report that he had been seen. That's natural and many bereaved people think they see their lost loved one from time to time.
The trouble was, the legend grew and Saul saw his opportunity. He climbed onto the bandwagon and ensured his place in history.
---
Now that may sound blasphemous but it's a whole lot more reasonable than the guff we're usually taught.
Dear Anonymous,
If you would not mind, I would like to approach your interesting Q as "why did this small group not fade out?"
1. Originally, those around Yeshua ben Joseph were a small, exclusively Jewish group inspired by a charismatic teacher. Then St. Paul really helped open the teachings to gentiles.
2. The amazing influence of Emperor Constantine; Christianity became the official religion of the Roman Empire, with even competing variations of Christianity stamped out, and lots of temples and money.
http://www.pbs.org/wgbh/pages/frontline/shows/religion/why/legitimization.html
3. The incorporation of ancient mythology proven to attract followers; the virgin birth, a founder who is a god, resurrection of the dead, even the December 25 birthday, and so on.