The reason xtians must 'prove it' to unbelievers is because one of the NT manuscripts state 'to preach the gospel to the world'.
IOW, you are to be ambassadors for your Jesus. If you can't provide the proof that the unbeliever asks for then you are not qualified to be a messenger.
BTW, Jesus was/is not god.
So, when did "preach" and "prove" became the same definition?
When did that happen? pray tell.