As historical novelists, we all must create characters who are recognizable to readers and yet remain true to their own times.
To be fair, there is no way we can completely shed our 20th/21st century selves. We are formed so profoundly by our world that even those of us who feel a deep affinity with another era cannot totally escape the one we were born into. That noted, however, we are obliged by the expectations of historical fiction readers not to redress modern people into period costume and call it historical.
Research into our chosen era is crucial. We need to understand how and why people thought the way they did. Some of that may feel so illogical or strange to us that we know readers will find it hard to sympathize with the characters we portray as positive. How to get around this? My answer is to remember the wisdom of my favorite grump, Ecclesiastes, who so famously wrote that there is no new thing under the sun.
In any modern period, people are inclined to believe that their era is wisest, full of fresh ideas, and better than any other. In fact, democracy in some form was present in Athens and even medieval monasteries. The confederation of the original twelve tribes of Israel, albeit under a king, resembles the beginning of the United States with the thirteen colonies. Are there differences? Yes, but there is a resonance.
The more original sources from the medieval era I read, the more I am struck by similarities between that era and many others, including this one. Western Europe may have been predominantly Christian, but it was also influenced by Islam in medicine, mathematics, language, and even spices. There were non-believers, gay people, powerful women, and Jews who were respected and accepted. The trap for historical fiction writers in recognizing this is giving their characters modern perceptions and rationales for the deviances from the conventional attitudes, also known as the era’s party line. What we cannot do is use our frame of reference and logic to present a point of view that proves the universality of human thought. What we must do is find the rationale our characters would have used in their time. For Sanctity of Hate, I found a letter by Pope Gregory X and a speech by Bernard of Clairvaux which argued that violence against the Jews was against the will of God. The rationale was not based in an understanding of the absurdity of anti-Semitism. It was based in the belief that the end of the world could not come if the Jews were killed.
So the answer to the problem lies in knowing how complex your chosen historical era was, recognizing the universality of human nature, and understanding the way in which your characters can rationalize their actions within some aspect of the logic from their time.