Unfortunately, many of us fail to recognize this. In fact, it has become quite fashionable to heap scorn on the backwardness of earlier generations, especially historical figures from those generations that are often held up as "great." For example, when Bernie Sanders was asked which foreign leader he took inspiration from with regards to foreign policy, he pointed to Winston Churchill because of his leadership in the fight against Hitler and the Nazis. For this, Sanders was excoriated by some on the left because as Sanders noted, Churchill was something of a conservative and pursued policies (both foreign and domestic) that reflected his colonialist upbringing. Another favorite target, especially among Mainline Protestants is the Apostle Paul. Although one can make a pretty strong case that for his time Paul was rather progressive, many of us Mainline Protestants can't resist demonstrating how much more enlightened we are than someone who lived 2,000 years ago.
I'd like to suggest an alternative approach to evaluating the "greatness" of historical figures, one that takes its cue from statistics. When researchers use statistical methods with samples of data in order to test the likelihood that an observed association between two variables (e.g., level of education and voting for Trump), they determine whether the observed association differs significantly from a scenario in which there is no association at all. Specifically, when the former is greater than 1.96 standard deviations from the latter, the difference is considered to be "statistically significant," which means that there is only a 1 in 20 chance that observed association is spurious. In terms of the graph below, when the observed association falls within one of the tails of the distribution (shaded in the graph), it is considered to be a statistically significant distance from the distribution's mean ( = 0).
What has this to do with judging individuals from the past? I would argue that we can consider those who differ significantly from the population mean on various issues as potentially "great." In fact, pushing the analogy a bit farther, we could consider those who differ in one direction from the population mean as dinosaurs, while those who differ in the other as enlightened (in statistics, these are known as "one-tail" tests). Easier said than done, of course, since it's a bit difficult to return to the past and conduct surveys. However, we can roughly "estimate" how much various individuals deviate from the mainstream of their day. And I would argue that approaching history with such a framework would help us be a bit more tolerant of those from the past. That doesn't necessarily mean condoning their beliefs and behaviors, but it might prevent us from judging them too harshly. And, if such an approach catches on, it might prevent future generations from judging us too harshly although that's probably just wishful thinking on my part. As Yoda might say, "Consider us dinosaurs, they will."