Christian changes to its predecessor, Judaism, included the reduction of 700-800 Jewish religious laws into two Christian laws: love everyone as yourself, and love God. But the biggest contribution of Christianity to modern religion was the concept of complete forgiveness.
The importance of this Christian rule goes to the nature of hate, the greatest challenge humanity faces. Real haters know no better and find it impossible to prevent themselves from hurting other people, and they are aware of this disability. They are constantly aware that retribution for the pain they have caused to others may be around the corner, and the paranoia that this fear causes, may raise their level of hate exponentially, hate for the people whose anger they have brought unto themselves; this is when things get really dangerous. Hate is caused by a lack of empathy, a class of disorders caused by broken or missing empathic neurons and related neural constructs. People suffering from a lack of empathy are emotionally isolated; they are distant from resources that can help with their situations. They operate based on misconceived perceptions of their place in the world because of their isolation. They blame others for their situation; their condition is very much like being on cocaine. Cocaine users, because of their mental isolation, are narcissists as were Hitler and Saddam Hussein, according to experts.
Mental isolation, paranoia, and narcissism go together. Another example of a paranoid individual is Richard Nixon. Nixon's actions personal isolation as the president are well known; his paranoia ultimately caused him to reveal his crimes. He suffered an exaggerated sense of self-importance, and believed himself to be a victim. Beck supports this description of this kind of isolated condition in his Prisoners of Hate.
Christ's approach of forgiveness diffuses the paranoia that hate causes; to forgive the actions caused by it and to reassure the paranoiac, reducing the fear the paranoia causes, reducing the chances of further destructive actions.
This in an unintuitive strategy; most of us, and nearly all natural organisms, react to a threat with a counter-strategy. As amazingly unlikely as that sounds as a strategy, I believe it is correct and works better than any other, and this is what Christ preached above all else: forgiving those who "trespass" against you, and loving your enemy as yourself.