I write at an important moment of remembrance and reminder, of bearing witness, and of action. Indeed, I write from Prague, where events commemorating the liberation of Auschwitz 70 years ago are underway as the world marks International Holocaust Remembrance Day. I write on the occasion of the 70th anniversary of the liberation of Auschwitz, the most brutal extermination camp of the 20th century, and site of horrors too terrible to be believed, but not too terrible to have happened.Of the 1.3 million people murdered at Auschwitz, 1.1 million were Jews. As Elie Wiesel put it, "The Holocaust was a war against the Jews in which not all victims were Jews, but all Jews were victims." I write also in the immediate aftermath of the 70th anniversary of the arrest and disappearance of Raoul Wallenberg on January 17, 1945. It is a tragedy that this hero of the Holocaust who saved so many was not saved by so many who could, and we owe a duty to Raoul Wallenberg to determine the truth of his fate.I write as well in the wake of anti-Semitic terror and killing in France, and in the midst of ongoing mass atrocities by Boko Haram in Nigeria, ethnic cleansing in Darfur and South Sudan, and killing fields in Syria and elsewhere. And so, at this important historical moment, we should ask ourselves: What have we learned in the last 70 years, and more importantly, what must we do?The first lesson is the danger of forgetting, and the imperative of remembrance -- le devoir de mémoire. As we remember the victims of the Shoah -- defamed, demonized and dehumanized as prologue and justification for genocide -- we must understand that the mass murder of six million Jews and millions of non-Jews is not a matter of abstract statistics. As we say at such moments of remembrance, "Unto each person there is a name, each person has an identity, each person is a universe." As the Talmud reminds us, "Whoever saves a single life, it is as if he or she has saved an entire universe." Thus, the abiding universal imperative: we are each, wherever we are, the guarantors of each other's destiny.The second enduring lesson is that the genocide of European Jewry -- like the genocides of Rwanda and Darfur -- succeeded not only because of the machinery of death, but because of a state-sanctioned ideology of hate. For example, the Jew was seen as the personification of the devil, as the enemy of humankind and humanity could only be redeemed by the death of the Jew. As the Canadian Supreme Court has affirmed -- and as echoed by the International Criminal Tribunal in Rwanda -- "the Holocaust did not begin in the gas chambers -- it began with words."The third lesson is the danger of anti-Semitism -- the oldest and most enduring of hatreds -- and the most lethal. If the Holocaust is a metaphor for radical evil, anti-Semitism is a metaphor for radical hatred. Let there be no mistake about it: Jews died at Auschwitz because of anti-Semitism, but anti-Semitism did not die. And as we have learned only too painfully, the killings in France being only one of the latest examples, while anti-Semitism begins with Jews, it doesn't end with Jews.The fourth painful and poignant lesson is that these genocidal crimes resulted not only from state-sanctioned incitement to hatred and genocide, but from crimes of indifference, from conspiracies of silence -- from the international community as bystander. Indeed, what makes the Rwandan genocide so unspeakable is not only the horror of the genocide itself, but that this genocide was preventable. No one can say that we did not know; we knew, but we did not act. Similarly, today, we have yet to act to stop the slaughter of civilians in Syria or the killing fields in Sudan, ignoring the lessons of history and mocking the Responsibility to Protect doctrine.The fifth lesson is the danger of the culture of impunity that repeatedly emboldens those intent on committing mass atrocities and genocide. Indeed, if the last century -- symbolized by the Holocaust and the genocide in Rwanda -- was the age of atrocity, it was also the age of impunity, with few of the perpetrators brought to justice. Just as there must be no sanctuary for hate and no refuge for bigotry, so must there not be any base or sanctuary for the enemies of humankind.The sixth lesson is that the Holocaust was made possible not only because of the "bureaucratization of genocide," as Robert Lifton put it, and as the desk murderer Adolf Eichmann personified, but because of the trahison des clercs -- the complicity of the elites -- including physicians, church leaders, judges, lawyers, engineers, architects and educators. Holocaust crimes were also the crimes of the Nuremberg elites.The seventh lesson concerns the vulnerability of the powerless and the powerlessness of the vulnerable, as dramatized at Auschwitz by the remnants of shoes, suitcases, crutches, and hair of the murdered, and as found expression in the triad of Nazi racial hygiene: the Sterilization Laws, the Nuremberg Race Laws, and the Euthanasia Program -- all of which targeted those "whose lives were not worth living." It is revealing, as Prof. Henry Friedlander points out in his work titled "The Origins of Nazi Genocide," that the first group targeted for killing were the Jewish disabled. It is our responsibility, then, as citizens of the world, to give voice to the voiceless and to empower the powerless, be they the disabled, poor, elderly, women victimized by violence, or vulnerable children -- the most vulnerable of the vulnerable.The eighth lesson is the cruelty of Holocaust or genocide denial -- a criminal conspiracy to erase and whitewash the horror of mass atrocity. In its most obscene form, Holocaust or genocide denial actually accuses the victim of falsifying the crime, of perpetrating a hoax. Thus, we have a responsibility to remember and bear witness to victims of the Holocaust and genocide, thereby repudiating genocide denial. The ninth lesson is the importance -- indeed the responsibility -- of remembering the heroic rescuers. Those "righteous among the nations," like Raoul Wallenberg, remind us of the range of humanity that prevailed in the face of evil and transformed history.Finally, we must remember -- and celebrate -- the survivors of the Holocaust, the true heroes of humanity. For they witnessed and endured the worst of inhumanity, but somehow found, in the depths of their own humanity, the courage to go on, to rebuild their lives as they helped build our communities. And so, together with them we must remember and pledge -- not as an idle slogan but as an injunction to act -- that never again will we be indifferent to incitement and hate, never again will we be silent in the face of evil, never again will we indulge racism and anti-Semitism, never again will we ignore the plight of the vulnerable, and never again will we be indifferent in the face of mass atrocity and impunity. We will speak up and act against racism, against hate, against anti-Semitism, against mass atrocity, against injustice, and against the crime of crimes whose name we should shudder to mention: genocide.Irwin Cotler is a Member of the Canadian Parliament, former Minister of Justice and Attorney General of Canada, and emeritus Professor of Law at McGill University in Montreal.