After thirteen hundred years from the time of Prophet Moses (P.B.U.H.), Allah sent Prophet Jesus (P.B.U.H.) to save the people from misguidance and sin.
The book that Prophet Jesus brought from Allah for the guidance of people is called the Gospel. Most of the people did not accept Jesus' invitation to the right path. They showed a lot of hatred and cruelty towards him instead.
Prophet Jesus stood firm guiding the people how to worship one Allah. He wanted people to love each other instead of hurting or being cruel.
On the other hand, some people with the help of the Governor of the city planned to kill Jesus. At the time in which it was planned to have Jesus killed, the guards and the soldiers of the Governor, by mistake, captured a different man who looked very much like Jesus and they crucified (hung) him.
Ever since, it is said that Jesus was crucified. In fact Allah saved Jesus from those wicked and cruel people. Christ is another name of Jesus and for this reason his followers are called Christians. The birth of Prophet Jesus marked the beginning of the Christian calendar. When we say 1999 A.D., it means that 1999 years have passed since the birth of Jesus Christ.