Christianity started when Jesus the Christ died on the cross of Calvary and resurrected three days later. Fifty days after Passover when Jesus was crucified on the Day of Pentecost the Church officially started.
Why did it start?
God in heaven is the Creator and sustainer of the universe (all things). The short version is that God wanted Christianity to begin.
Why? God created man and woman and put them in the Garden of Eden. He created them for Himself. They sinned and man and woman died spiritually. They were separated from God at that moment. Every one born since that time has been born spiritually dead and separated from God. Down through the ages since then God has begun a long process to bring man back to Himself.
He started with Abraham in order to begin a new nation, Israel, one that was separate from the rest of the world and through this nation God brought fourth the Messiah (Jesus Christ).
Approximately 2000 years ago Jesus the Christ (Messiah) lived on the earth having come down from heaven. He died on the cross as a sinless man in order to pay for the sins of man and provide a way for man to come back to God. Jesus was the God/ Man, fully God and fully Man
Even though Jesus Christ died for man's sin, righteousness is not imputed automatically to man. Man has to accept Jesus Christ as His personal Savior in order to be saved from the wrath to come.
The Bible plainly tells man that Jesus Christ is the Savior and that there is no other way to approach God that through Him in salvation.
Judaism & Christianity started with God's Covenant with Abraham.
Jamestown
The ideas of Christianity started in Judea with the teaching of Jesus Christ.
No it did not. It started in Judea.
Israel.
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