Many skeptics claim that we can't trust what the New Testament authors said because they're bias. They said that they have an invested interest in writing down the things they wrote down. They say it's propaganda. It's a religious text. It's meant to be a tool for converting people and nothing more. Are they write? Can we really not trust what the gospel writers tell us because they're "biased" or because they're interested in converting people to Christianity? Is this really a good reason to throw out the gospels when trying to find out information about the life of Jesus? I don't think so.
For one thing, everyone is biased. Everyone has an invested interested in what they're writing. For example, when we read Gerd Ludemann’s book “The Resurrection of Christ” a former Christian, now an atheist New Testament scholar, and in his book “The Resurrection Of Christ” he says “It’s aim was to prove the non-historicity of the resurrection of Jesus and simultaneously to encourage Christians to change their faith accordingly.” (by the way, Gerd Ludemann believes Jesus existed) Do you think Ludemann is bias? Does he have an agenda? Yes. Does that mean we should throw out everything he says? No. He could be bias and he could be right.
What about Richard Dawkins? In his book “The God Delusion” he writes “If this book works as I intend, religious leaders who pick it up will be atheists when they put it down.” Do you think he’s bias? Do you think Dawkins has an agenda? Should we throw out everything he says? No. At least you should be consistent if you’re going to do this. But it’s really dumb if you do that anyway. I mean, Jews have an invested interest in writing about the holocaust (namely to try to prevent such an atrocity from ever happening again), blacks have an invested interest in writing about the unfairness of slavery, so rejecting what a document says because they’re supposedly bias is just fallacious. Basically, it’s just an ad hominem (rejecting what a person says as true simply because of who they are).
Are the gospel writers biased? I guess so. But not more biased than anyone and everyone else. But since people do sometimes distort the facts because of their bias, how can we know for sure that what the gospel writers tell us are true? Simple. We can get passed biases by examining what the documents say using historical principles of authenticity such as the principle of embarrassment, the principle of multiple attestation, the principle of early testimony, the principle of dissimilarity, etc. These are historical tests that historians apply to ancient documents in their attempts to discern whether what they say is true or not. While these historical tests can't be used to prove that something in a document is false, it can be used to prove that something in a historical document is more plausibly true than false. What I mean by that is that just because something is not embarrassing, or not multiply attested doesn't mean that it's false. It just means that if something IS embarrassing or IS multiply attested, we have more certainty that it's true than we would have had in the absence of such details.
We can also see if they check out in other seemingly insignificant details. The gospels get a lot of what most Bible readers of the 21st century consider to be unimportant details. Colin Hemer wrote a book called "The Book Of Acts In The Setting Of Helenistic History" and Craig Blomberg wrote a book called "The Historical Reliability Of The Gospel Of John". In these books, the authors list dozens and dozens and dozens of historical details that the New Testament writers got correct. 84 details in Acts have been confirmed and over 50 details in the gospel of John have been confirmed by Archeology. For the sake of brevity, I can't go into any of these details right now, but if you want to learn more, go read those books. Anyway, the fact that the gospel writers (and the writer of Acts) get SO MANY details correct, we have good reason to believe that they were eyewitnesses to the events they wrote down. It would be impossible to get that many details right if you were a person living in another place at another time.
Moreover, just because someone is bias doesn’t mean they’re wrong. As Mike Licona pointed out in a lecture I heard a while back, bias can actually drive people to be MORE accurate in their details because they want you to believe what they're telling you so they don't want to risk telling you false information even in minor details. They don't want their own words to discredit them. I do this when talking with non-believers on the internet. If I'm telling a non-Christian something and I'm not sure about what it is I'm telling them, I'll do some internet research to see if my information is accurate. I don't even want to falsely attribute a quote to somebody if it was somebody ELSE who uttered it. So, at least in my personal circumstance, my bias actually drives me to be more accurate in what I'm saying...because I don't want to hurt my witness by saying something that isn't true.
Reasoning the way many atheists do (that you should reject something because it's biased propaganda), you should probably reject everything Charles Darwin said in his book “The Origin Of The Species” because he had an invested interest in getting his theory off the ground. I should probably throw out everything any atheist has ever said in his blog because he's biased. But I’m guessing they probably want me to give them a fair hearing, right?