How can you trust the writers of the gospels since they were biased?

"The stories about Jesus were written by Christians therefore they are not valid, because they had an agenda. They were biased."
At first this sounds like it has some weight. However, I would compare it to this:
All the biographers that have been written about Abraham Lincoln were Americans so they are biased. Therefore they must not be trusted.

Of course no one would have written about him if they weren’t Christians. The Jews of First century 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.
So of course unbelieving Jews would want to ignore him. note: that there are extra biblical references to Jesus by an unbelieving Jewish historian. Although Christ Mythologists will claim it’s entirely plagiarized.
It’s like saying, “I can’t believe the stories you tell about your late father, because you’re his son. You loved him, so you must be biased. Therefore you must be lying.”
When in fact, I should only believe you are a liar if you have proven yourself to be a liar.
What’s more, let’s not forget that the people who said they were eye witnesses of Jesus DIED to prove that they had not made up the story.
(those of the Christ Myth camp will claim these church history documents are forgeries as well.)
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.