Bernard had long been an advocate of the crusading efforts in the Holy Land. As early as 1140 he had written in praise of the Order of the Knights Templar and their efforts to defend Jerusalem and other sites. The Muslim conquest of Edessa in 1144 caused great concern for many christians in the West, prompting the calling of the Second Crusade by Pope Eugenius in December of 1145 in his bull Quantum Praedecessores.
Bernard quickly became one of the most energetic and effective preachers of the crusade. Reportedly, in the wake of his preaching entire villages would be depopulated of all their able bodied males, who had taken the cross and were headed for the Holy Land.
Interestingly, Bernard made an effort to prevent the type of violence that was carried out against the Jews as in the First Crusade. As early as the 1130's, Bernard had been preaching against injustices done to the Jews and continued to express such sentiments during his preaching of the Second Crusade. As in the First Crusade, violence broke out again in Mainz which was incited by the preaching of a monk named Raoul.
The monk Raoul had argued that if they were going to fight for the faith then the logical first step would be to kill the infidels in their midst. The Archbishop of Mainz contacted Bernard to come to Mainz and help him before it was too late. When Bernard arrived, he began an intensive preaching campaign against Raoul and was so effective in shutting down violence towards Jews in this area the Bernard was considered by the Rhineland Jews to be a "righteous gentile."
Bernard also preached against the Cathars in what is today the south of France. Bernard famously went to the town of Albi on a preaching mission against the Cathar heresy. From this mission, people began to associate the city of Albi with the Cathars even though Albi became a stronghold of orthodox Catholics. As a result, the term Albigensian became interchangably used with Cathar to describe the members of the Cathar movement. For example, the term Albigensian was used by the chronicler Geoffroy du Breuil of Vigeois to describe the Cathars.
The Second Crusade ended in failure and was widely viewed as a failure. As a result widespread criticism of the crusading movement developed following its unsuccessful conclusion. Even St. Bernard, the most popular and beloved saint of the twelfth-century was not spared from such criticism. He wrote a well known Apologia for the Second Crusade in which he defiantly defended his preaching of the expedition and blamed the failure of the crusade on the sins of Christians.