Ivanhoe may appear to be a straightforward story of swords, knights, and drama (you know, all that cool medieval stuff), but there are multiple characters, both good and bad, with a very strong commitment to the rules, even if we don't always agree with those rules.
Look at Lucas Beaumanoir. He is a complete meanie, but he also insists that the Knights Templar practice what they preach, that they stick close to the vows of their order. Beaumanoir may be a bigot and a jerk, but man, he walks the walk. Speaking of people with principle, what about Rebecca? Her greatest virtue is her dedication to principle: it's the strength of her beliefs that keeps her safe from Brian de Bois-Guilbert's temptations.
We have principled characters on opposite sides of the good/evil divide: Beaumanoir is one of the book's worst villains and Rebecca is (in our humble opinion) the novel's best character. According to Ivanhoe, loyalty to principle doesn't necessarily make you a good person in and of itself. If you are dedicated to a moral code that is fundamentally unjust, then you will also be unjust, no matter how principled you may think you are.
When faced with the choice of converting to Christianity or dying, Rebecca refuses both to change her religion and to fight back violently against her oppressors. This supports the view that passive resistance is the best response to widespread prejudice.
All of the knights in Ivanhoe, from Brian de Bois-Guilbert and Lucas Beaumanoir to Ivanhoe and Athelstane, refer to abstract ideas of honor and chivalry. The fact that honor and chivalry appear to mean different things to each of them demonstrates that these concepts have very little bearing on the events of Ivanhoe.