"There is nothing new under the sun." Every generation likes to think they are more progressive than the previous generations, but they really are not. Although technology has increased dramatically in the last one hundred years, the human condition is still the same when it comes to beliefs. We may have more information than we had before but people are still people. Understanding the nature of history is crucial to having beliefs that are robust and true. There are two aspects that are especially noteworthy, context and consensus.
It is important not to take a historical event out of its context. Understanding the different nuances of the culture that you are in is difficult enough, let alone trying to understand a culture hundreds of years ago. For example, making definitive statements that Christianity was the cause of the Crusades may be taking things a bit to far. Did the United States invade Iraq because of oil or because there was a terrorist threat? Even though this happened in our lifetime there is little agreement as to the cause of the invasion. We can know that a particular thing happened in history, but it is a lot more difficult to know exactly why it happened. Another aspect of context is that it changes. Being in Christian circles, I often hear about how ineffective street preaching is and that nobody wants something shoved down their throat. Although that may be the context now, that does not mean it was the context then. In fact it would seem that street preaching must have been more effective then whatever we are doing now, since Christianity is on the decline.
Many people use history to demonstrate how they are better than people who came before them. Unfortunately the people who came before them are dead and cannot show otherwise. In any case, history should not be used to compare, it should be used for consensus. Comparisons are not good because of the limitations of understanding context, but finding consensus is good because it is context agnostic. If every generation believed it was wrong to kill your neighbour, being the first generation to believe that it is not wrong would not be a good thing. If the same belief is held independently of context it is a good indicator that the belief is solid and it is your context that is not.
Historical context and consensus are two great beacons to navigating one's beliefs. It will enable you to stay in the channel and avoid shipwreck. Before condemning previous generations for things that they did, take the time to understand the context. Instead of thinking that you are the first generation to finally get it right, consider that you could be the first generation to get it wrong. Learn from previous generations instead of comparing yourself to them because that leads to pride, which makes you think you are right, instead of being right.