Where did Christianity start and where did it diffuse?

Christianity started in Israel.

Another View:

the answer to the question "Where did Christianity start?" is that Christianity was started by Paul, at his headquarters in Antioch, in order to salvage his career as a missionary for the Jewish sect that Jesus had started. Paul, in the 17th year of his career was faced with choosing between retaining his members or else repelling them by demanding them to go under the knife in an era which lacked anesthesia, antibiotics and antiseptics. He chose to retain his members, even though he knew that by his abandonment of the Jewish covenant he was teaching the opposite of what Jesus had taught. The authors of the four Gospels were Paul's followers, who manufactured a "Jesus" that embodied Paul's agenda, not the agenda of Jesus.

Another Answer:

Ten days after Jesus had ascended into Heaven, on the first Pentecost or 50th day after His Resurrection, God poured out His Holy Spirit upon the disciples of Christ in Jerusalem. Some have dated this to be Pentecost Sunday, 17 April 31 AD.. Immediately, Peter speaking to the crowd gathered, told them to repent and be baptized to follow Jesus (see Acts 2:38-41). Thus began Chrisitianity.

Shortly thereafter, Peter escaped from those looking to persecute him. He went to Babylon and set up headquarters. The incipient Church center remained in Jerusalem and overseen by James - Jesus' half brother. The remaining Apostles began to reach out to the dispersed Jews throughout the Roman Empire. Paul was converted and given a special task to go to the Gentiles, Kings and the children of Israel - commonly referred to as 'the Lost Ten Tribes' - (see Acts 9:15). And so the difussion began...