Because Jesus rose from the dead. 500 people saw him after the Crucifixion. 1 Corinthians 15:3-7 is the first Christian Creed which is a formulaic written form which is a learning device. Think of a Creed like you would have sayings in school as you are studying to remember things.
You are why people are leaving organizeed Christianity. The intolerant demand that your religion is the only one that counts.