When someone makes the claim (i.e. The Holocaust did not happen; the Moon Landings were staged; 9/11 was an inside job), the person making the claim must be able to provide sufficient evidence to support his positive claim. Just like in a court of law, the defense does not need to support evidence that they are innocent; the claim is made by the Plaintiff and they must provide sufficient evidence. When we don't do this, we are subject to our cognitive baises and beliefs, which could distort the evidence.
Hence, just because we believed the world was flat, did not mean in the face of evidence that it was. Did everyone change their mind instantly? No, but some did not wish to see the evidence either.
We are conditioned by our cognitive biases to refute evidence that goes against our beliefs. In fact, there are parts of your brain that light up in the same areas of disgust when something challenges your belief systems.
Hence, this is why racism, sexism and homophobia, while are illogical fallacies of belief, take so long to change in people's minds. But we can provide conclusive evidence that these beliefs do not match a well tested hypothesis of equality, and that there is no reason logically or morally to suppress others in our behaviors or actions.
So when systems of religion are challenged, there seems to be a sense that somehow these ideas are off limits. Yet when we look at the news, we see that the majority of racist, sexist, and homophobic people in our societies tend to lean to the religious right. It does not mean that all religious people are racist, nor are all people on the right. This is why we see terrorist attacks. It doesn't matter if not all people are that way. The fundamental belief system is within the very books themselves. Are these the works of Gods, or are the not?
The point is that where are the ideas originating from? If we can show you that there isn't a difference that warrants this need for racism, sexism, nor homophobia, the question you must ask is where can you look to find it?
Well, one place is in the Bible. If someone believes that this book is the true word of God, and there are verse from God, which include slavery in it, when we know that slavery is not acceptable today, the question arises why we think this book is a good representation for our society?
The truth is the average person has not read the Bible, nor subscribes to all the teachings. A large percent do not think most of it is literal. Yet we can't seem to pinpoint where Christianity actually lies now without all the confusion, yet they seem to think they have enough knowledge to tell our educational systems about evolution and women's rights.
And if I can criticize other bad beliefs such as conspiracy theories and UFO cults, I am not sure how religion gets a free pass. There are a lot bad ideas within religion. It is okay. Most people act like they made up the religion themselves. They didn't. But you would be hard pressed to find people taking the time to sit down and figure out why it is actually being criticized. It does no good to anybody unless you are objective in your findings to understand a claim when you make it. But you can't expect others to just respect a belief because you have one. That is just not how this world operates.