Why? It's simple, Christianity doesn't exist because of the Bible, the Bible (the New Testament specifically) exists because of Christianity. The Bible exists because people witnessed something that happened that was worth documenting. The Bible exists because within our human history this man named Jesus enters our world, says some crazy things, then backs up those crazy things by rising from the dead after he was crucified.
Jesus is crucified around 30AD, three days later he rose from the dead and the church was born.
On August 6th, 70AD the temple in Jerusalem is destroyed by the Roman legions. That historic event and others before it and surrounding it are not recorded in the documents of the New Testament because they had not happened yet. This fact tells us that the dating of all of the New Testament writings is before this time. This puts the dating of all the New Testament documents earlier than 70AD. And the earlier they are the more reliable they are.
The writers of the New Testament were not writing what they believed, they were recording what they saw. They were documenting historical events regarding the birth, life, death and resurrection of Jesus.
Once Constantine rose to power as the undisputed emperor in 312 AD, he needed to find something that would unite the empire. He chose Christianity. Under severe persecution Christianity grew exponentially leading up to Constantine's rule.
This happened before anyone held a Bible in their hand. The 1st, 2nd and 3rd century Christians believed Jesus loved them before the Bible told them so. For the first 300 years of Christianity’s existence the debate was not centered on a library of books called the Bible, the debate centered on an event.
The question wasn’t “is the Bible true” the question was “did Jesus rise from the dead?” And all the evidence well documented by those first century witnesses and writers point to a resounding yes. For people like John, Peter, Luke and Paul the unequivocal answer was "yes" and they were willing to die to for their "yes."