When was Christianity founded?
This is a great question and there are generally speaking two schools of thought. I really depends on who you ask and his or her persuppositions are.
First, most people will say that Christianity began with the teachings of Jesus, his death and resurrection, and the teachings of the early church. This would make the formation of Christianity in the 1st century A.D. The New Testament would be some of the earliest documents. To be sure, there has been lots of developments and transformations, but the origins would be put during the 1st century.
Second, some people would say that Christianity dates back to the Old Testament. They would say that Jesus was the promised messiah. So, Christianity did not start with Jesus, but with the one who gave the promise, namely, God. They would trace this back to Genesis 3:15 and perhaps more importantly to the Abraham covenant from Genesis 12-17.
In the end, it really is a matter of faith.