Christianity started as Christianity. Christianity did not develope from some other belief into what it is today.
People were first called Christians in Antioch, probably this was at first a term of reproach.
The disciples (meaning imitators) of Jesus Christ began to be 'called out' on Pentecost Sunday, June 17, 31 AD, outside of the Upper Room in Jerusalem. They referred to themselves as 'The Way' or followers of the Nazarene - primarily Jewish but rapidly incorporating gentiles as well - the 'little flock.'
It wasn't until Antioch some 10 years afterwards, that the 'derogatory' label of Christians - followers of Christ - was first used. This term has become the dominant term since for the over 30,000 groups/churches calling themselves 'Christian' each with some difference in structure or interpretation of the Scripture. Jesus, however, began one 'Church of God' as a home and training area for those called out in their lifetimes to follow Christ and do God's works (see John 6:44, 65; and Ephesians 2:10). During the Millennial Reign and the Judgment process to follow, the rest of mankind - the greater harvest - will be called and given the opportunity the 'firstfruits' have in their lifetimes.
Judaism & Christianity started with God's Covenant with Abraham.
Jamestown
The ideas of Christianity started in Judea with the teaching of Jesus Christ.
Israel.
No it did not. It started in Judea.
Christianity and Islam.
That's like asking why did Christianity start in Jerusalem?
Christianity can be said to have started with Peter's preaching on the Day of Pentecost, in AD 30, in Jerusalem.
Christianity means followers of Jesus Christ. After the passing on of Jesus, the Apostle came together to continue the work. This was the start of Christianity in Jerusalem.
Some have dated the start of Christianity to the 1st Pentecost in Jerusalem on Sunday, June 17th, 31 A.D..
Christianity acknowledges that Judaism began with Abraham as the first person after Noah to have a personal relationship with God.
Christmas was brought to France when the population converted to Christianity