The following was written by James Warner Wallace founder of the apologetic organization “Please Convince Me.” He provides excellent answers for anyone making the claim the Biblical writers were biased.
Objection: There is insufficient non-Christian historical evidence to substantiate that Jesus lived. All the supposed eyewitnesses of the life of Jesus were Christians. They cannot be trusted because they were biased.
The Answer Lies in the Eyewitnesses Themselves
While there are ancient non-Christian sources (Josephus, Thallus, Pliny the Younger, Suetonius, Tacitus, Mara Bar-Serapion, and Phlegon, to name a few) who describe the existence of Jesus and the life of those who came to follow him, the best and most thorough eyewitness accounts of the life of Jesus are the New Testament Gospel accounts of Matthew, John and Peter (written through his scribe, Mark). The testimonies of these eyewitnesses are often disregarded, however, on the basis that they were written by “biased” Christians. But the Gospel authors are reliable according to the current standards we use to determine the validity of eyewitnesses:
The Eyewitnesses Didn't Start Off With a Bias
None of the three authors started off as disciples of Jesus. While John and Peter were disciples of John the Baptist, they clearly expressed doubt about the identity of Jesus and this doubt is recorded in the Gospels. In addition, Matthew was a tax collector and knew nothing about John the Baptist's teaching. The eyewitnesses didn't start off with a bias.
The Eyewitnesses Only Became Christians in Response to What They Saw
While it is true that the Gospel writers eventually became believers, it is illogical and irresponsible to disregard their testimony. Imagine that two witnesses observe a liquor store robbery. They recognize the robber (from their neighborhood) and later tell police that it was John Smith. Both started off without a bias when they entered the liquor store. But as a result of what they saw, they now hold the position that John Smith is a robber. One could say that they are now biased; after all, they now hold a distinct view. But it would be illogical to disregard their testimony and demand a new set of witnesses who don't hold a belief that John Smith is the robber! The belief of the witnesses was shaped by what they saw, NOT by what they believed BEFORE the robbery occurred. In a similar manner, the eyewitness Gospel writers formed a conclusion based on what they observed.
The Eyewitnesses Are Reliable in Other Areas
The Gospel writers record more than the life of Jesus. Their account of First Century life in Palestine is geographically and historically accurate in areas that have nothing to do with Jesus. The Gospel of Luke (and Book of Acts), for example, have been tested by many skeptics, only to survive as remarkably accurate historical records.
The Eyewitnesses Had Little to Gain by Lying
One thing is for sure, the eyewitnesses all went to their graves (most in a dreadfully painful way) without recanting their eyewitness testimony about Jesus. The witnesses never gained anything in the way of money, sex or power from their testimony, and there is NO ancient record that records anything other than hardship and martyrdom for these eyewitnesses. They clearly stood by their observations in spite of the liabilities.
The Gospel eyewitnesses are reliable in spite of the fact that they eventually became Christians. They were not biased; they were simply convinced.