Over the last several years I've come across a couple of "philosophies" when it comes to how a person's personal relationship to a belief affects their ability to represent it accurately. I have heard people state that if someone does not believe something, then they cannot accurately represent it. The "support" provided is that if they were familiar enough to represent it accurately (and in its most powerful form), the person would believe it. But then I've also heard many say that those who believe something cannot accurately represent it. The "support" provided is if someone believes something, they obviously want it to be true, so they will misrepresent a false belief in order to support its truth.
What's interesting is that I've heard both from a couple of the same people, but with regards to different topics: "You can't trust the disciples of Jesus to tell us the truth about him because they obviously wanted to believe he was God and the resurrection happened," and "You can't trust a Christian who used to be an atheist (or other worldview) to accurately represent atheism (or other worldview) because he does not want it to be true." The only identification of when to use which is, "whichever one supports my own view." But that is not a valid reason because of its subjective foundation. When I apply either critique is dependent totally on me, and I can change it whenever I please.
The problem is that there is no objective dividing line provided (much less, evidence for the line) to determine which of the claims for reliability to apply when, where, and to whom. It appears, then, that we must apply one of them consistently across the board. Unfortunately, it doesn't matter which one we choose to apply consistently to everyone at all times. I want to try two things. First, let's attempt to apply either of these to other people.
The first critique would rule out the possibility of people who are experts on Greek mythology from being able to tell us accurate information about it. Just because someone does not believe something, does not prevent them from presenting it accurately. The second would rule out the possibility of trusting locals who believe that they experienced an earthquake.
Now let's apply each of these to ourselves. We either believe something or we don't (I'm not talking about our level of certainty). If we believe it, we can't trust our own support offered for it, and if we don't believe it, we can't trust our own support offered against it. That we can't trust ourselves or anyone else takes us to a complete skepticism of everything, including the reasons for the skepticism itself.
It doesn't matter which one is used, problems exist. This is why neither should be used. All claims need to be evaluated based on the claim itself, not where (or who) the claim comes from. In formal logic, this kind of argument is called the "genetic fallacy,"- that is attempting to establish a claim's truth or falsehood solely based on the source of the claim.
It is true that people do misrepresent claims because they want them to be true. It is also true that people misrepresent claims because they want them to be false. But those are not the reasons that we should dismiss the claims. We need to evaluate the claims themselves. Evidence creates bias towards and against claims. If we evaluate the evidence rather than making irrelevant accusations, we might understand the reasons why someone has chosen to believe or not to believe something. If the evidence further convinces us of the truth or falsehood of a claim, we can add it to our personal library of evidence. If not, then we need to be able to understand and articulate the reasons that we are not further convinced by the provided evidence of the claim that convinced the other person. Then that may be added to our personal library. Both of these can be used to increase or decrease the level of certainty about the truth or falsehood of a claim.
My point is that making a claim of support or denial based on the source is not going to get us anywhere intellectually. It doesn't allow us think any deeper about our beliefs, and may even have a negative affect on our credibility to those who are honestly searching for the answer. On the other hand, if we skip arguments that commit the genetic fallacy, we have the opportunity to engage the evidence; and if we engage the evidence, we grow intellectually and our credibility to those searching will be high..