Biased Writers

I've often heard some form of this statement from atheists. “The writers of the gospels were biased. Therefore, The stories about Jesus can't be trusted because the writers had an agenda.”
At first, this sounds like it has some weight. However, I would compare it to this hypothetical statement: "All the authors who wrote about how awful slavery was, had an agenda. They wanted to see slavery abolished, so they can't be trusted because of their bias." We would agree that those writers were biased, but that doesn't mean they were lying.
Or it might be like saying, “I won't believe you when you tell me that your late grandfather was a good man. You loved him, and you want me to love him too. You're biased therefore you must be lying.” Of course, there is bias when you love someone, but it doesn't mean you're lying. 
No one but believers would have written about Jesus in the first century. The Jews of Palestine just wanted to pretend it didn’t happen. Because Jesus’ existence put them at great risk of being crushed by the Romans. They had him executed, which points to how much they wanted to silence his story. The Romans executed thousands of criminals every week. They had no reason to write about him until decades later. The only people that had a vested interest in telling his story early on were those who had become believers.
There are a handful of references to Jesus that are outside the Bible, but they come from a number of years after his life and ministry.
There's no reason to think that the eyewitnesses were liars and quite a lot of evidence that suggests that they were telling the truth. The people who said they were eyewitnesses of Jesus were executed in horrible ways. They accepted this fate to prove that they had not made up the story, and as an expression of their faithfulness to Jesus. All they had to do was renounce their Christianity and worship Roman gods and their lives would have been spared. We know they could have spared their lives because Emporer Trajan said so in a letter to Governor Pliny around 100 A.D. Trajan said, "whoever denies that he is a Christian and really proves it--that is, by worshiping our gods--even though he was under suspicion in the past, shall obtain pardon through repentance." We wouldn't have blamed them for renouncing to spare their lives, even Jesus would have forgiven them. However, they stood strong and proclaimed that their testimony was true. Those that claimed to be eyewitnesses of Jesus chose death. They were utterly convinced. 
If you want to know what Jesus was like who should you ask, an unaffiliated person who has no opinion about him, or the men and women who were so impressed by him that they were willing to die? The unbiased person may be able to give you some general background and history, but it's the biased people who can tell you what he was really like. I'd choose the biased people, as long as I'm confident they weren't lying. 
For anyone who proposes that the gospels can't be trusted because the writers were biased, I'd say, "Yes, they were biased, but that doesn't mean they lied, show the proof that the disciples lied." Oh, there is no proof, just speculation. It turns out the speculation is extremely biased. We do, however, have a host of statements in which the eyewitnesses claim that they are telling the truth. Until we have proof that they were lying, it's most reasonable to believe they were telling the truth.
Did early believers have an agenda? YES, of course, they did. That does not mean they were liars. They died to prove they weren’t. So, it turns out that bias is not a bad word when it comes to the gospels.