Given that the copies of the Bible are faithful to the original, how do we know that the original itself was telling the truth about an actual event?

One of the criteria is to see if there are some cross-references from outside sources that corroborate the same event or situation. There are numerous other non-biblical sources and historians that recorded the events surrounding Jesus and the early church.

The Jewish Talmuds, which were written by the enemies of Jesus, do not deny that Jesus performed signs and miracles, although they try to explain them away as demonic. Additionally, Roman and Jewish historians were both critical of Jesus, yet they still acknowledged his existence and impact.

The Encyclopedia Britannica (15th edition): “These independent accounts prove that in ancient times even the opponents of Christianity never doubted the historicity of Jesus, which was disputed for the first time and on inadequate grounds by several authors at the end of the 18th, during the 19th, and at the beginning of the 20th centuries.”