Updated: Oct 19, 2021
There is an argument that I see raise its ugly head every now and then that (I am ashamed to admit) I used myself before I was a Christian, and I really thought it was a powerful one. It goes something like this- when Christians try to defend the origins of their faith and the historicity of the resurrection they rely heavily on Christian sources (the bible, writings of early church fathers etc). To me, that was biased- because of course those sources are going to say that Jesus rose from the dead or that there were really miracles. After all, they wanted to make people believe what they believed.
I refused to accept these sources, because as far as I was concerned only non-biased (i.e. non-Christian) historical sources should be consulted. That way we would know they were telling the truth as they had no agenda. It never occurred to me that my view was biased. To my way of thinking, any Christian source or testimony was automatically suspicious, and could be ignored. It doesn't take much effort to see how flawed this thinking is.
Firstly, consider what would happen if there was a trial and the judge said "I am going to ignore all of the eye witness testimony of the people who say that the defendant is innocent- they are clearly biased. I just want to hear the evidence from the prosecution, and I will base my decision on that". There would be uproar, and rightly so. But this is similar to the tactic employed by people who use this argument- they are in effect saying "I am only going to listen to the views of the people who agree with me because they are the only ones that can be trusted. Then I am going to make my decision based on that evidence, and anyone who disagrees is clearly wrong!"
There is no other sphere of human knowledge where this would be accepted, but plenty of atheists consider it to be a perfectly valid argument. Imagine their response if you said at the outset of any discussion about faith or the bible "The only sources or authorities worth considering are Christian sources, and anything any non-Christian says should be discarded because it's clearly biased!" This would, of course, be met with anything from stunned disbelief to outright anger, but it is the same argument that they would be using themselves.
Much more fundamental, though, is the one thing that never crossed my mind when I was an atheist. I demanded that only non-Christian historical sources would be reliable evidence to support the things in the bible- things like the miracles and resurrection of Jesus, and the incredible things that happened in the early church as Christianity spread like wildfire. I wanted to hear evidence from someone who was not a Christian that supported the teaching that Jesus of Nazareth had healed the sick, cast out demons, raised the dead, predicted His own death, and was crucified (in accordance with His predictions), then rose from the grave three days later, spent 40 days teaching and preaching to crowds of people, and then ascended to heaven. It never occurred to me that if someone had witnessed these things, or at least believed them to be true because of the eyewitness testimony of dozens (or even hundreds) of other people, how on earth would they then not become Christians? If I had lived at the time of Jesus, and had witnessed these things or at least had been presented with enough evidence to be convinced of their truth, would I then produce some kind of cold and distant report of it and then carry on with my life as if nothing had happened? Of course I wouldn't, and I can't imagine that anyone would.
We should, of course, be careful and considered in any kind of historical investigation, and should weigh the evidence as objectively as we can, taking whatever steps we can to ensure that the sources can be trusted. In addition, evidence that contradicts our position must be given a fair assessment, and if necessary contradictory evidence should cause us to examine our view.
But to insist that only non-Christian sources are reliable and to simply reject Christian sources shows a level of bias that would not be accepted in any other discussion, and should not be accepted in the examination of Christianity.