Imagine that three people perform the exact same way in a singing competition. You are then asked to evaluate these three people. Would you give them identical evaluations? Would others?
There are dozens of factors that could influence these evaluations that have nothing to do with how each person actually performed. For instance: What does each person look like? Is the person male or female? What race is each person?
But, I would like to focus on what we know about the person going into the performance. Imagine, for instance, that we know one of the people holds values that are starkly different than our own. Research suggests that we would evaluate how this person did more negatively, even if the belief differences have no real relation to the performance. The same would be true if we heard a friend say something negative about this person before judging them; we would again tend to evaluate the person worse than we would have otherwise.
So for instance, if you had a boss or college professor that you found out had different political beliefs than you, that professor or boss would have to do a better job to get you to think of them favorably than if they held the same political beliefs as you. The same goes for any belief really.