When did Christianity originate? How long has Christianity been around? Not how or why -- just when did it start:


As a religion?
As a name?
1. Christianity as a religion

The central point of the whole Christian faith is the resurrection of Jesus Christ:


  1 Corinthians 15:19-20 (ESV)
   19If in Christ we have hope in this life only, we are of all people most to be pitied.
  20But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep.   


This happened ~30 AD, that is 33 years after Jesus's birth, the exact year of which isn't certain.

But the central point is not the same as the beginning. Hebrews 11 tells us that the patriarchs of the Old Testament era were already waiting for Christ, living in the hope of the promise.


  Hebrews 11:24-26 (ESV)
   24By faith Moses, when he was grown up, refused to be called the son of Pharaoh’s daughter, 25 choosing rather to be mistreated with the people of God than to enjoy the fleeting pleasures of sin. 26 He considered the reproach of Christ greater wealth than the treasures of Egypt, for he was looking to the reward.   


The Old Testament era was building up for the coming of Christ (John 1:1-18, Romans 5). When Adam and Eve originally rebelled against God, the plan to save humankind began. What we call Christianity is really that plan. Adam, the first man, already is a symbol of Christ:


  Romans 5:14-15 (ESV) [emphasis mine]
   14Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.
  15But the free gift is not like the trespass. For if many died through one man’s trespass, much more have the grace of God and the free gift by the grace of that one man Jesus Christ abounded for many.   


It makes sense to say that Christianity began at the Resurrection, ~30 AD, but it makes even more sense that Christianity began at the Fall of man.

2. Christianity as a name

The first mention of Christians was of the group of believers in Antioch:


  Acts 11:25-26 (ESV)
   25So Barnabas went to Tarsus to look for Saul, 26and when he had found him, he brought him to Antioch. For a whole year they met with the church and taught a great many people. And in Antioch the disciples were first called Christians.   


This was obviously after Jesus's resurrection, but also during the reign of king Herod Agrippa (Acts 12:1 mentions this). From Herod Agrippa's period of reign we know this was probably between 41-44 AD.