Who invented Christianity?
In Acts 11:26 the Bible states that the disciples (or followers of Christ) were first called Christians in Antioch. So I'd have to say that the term was established there.
On a basic level, though, Jesus Christ himself started the whole ball rolling around three and one half years before his death.
It has had many variations in outward form and substance over the years, but even after 2,000 years, we still take the Bible as our guidebook. So should you.