In October 1939, Trzeciak was a founding member of the pro-German, anti-Soviet, and antisemitic National Radical Organization. After the Easter pogrom, however, he decided to form a more moderate faction.[citation needed] During the German occupation of Poland in World War II, he maintained friendly relations with the Germans, which he used to help his friends.[4] Thanks to his friendly relations with the Germans, he managed to save colonel Ignacy Oziewicz from the camp in Kulautuva, Lithuania. His antisemitic articles were published by occupying forces and quotations from his books were placed on Nazi posters.[3]
According to the reports by the Home Army, Trzeciak was a collaborator with the Gestapo.[3] On 22 March 1941, he denounced Tadeusz Puder, a priest of Jewish origin. As a result of denunciation, Puder was arrested on charges of not carrying an armband, which, according to German decrees, should be worn by all Jews. On September 1, 1941, he was sentenced to one year and eight months in prison.[3][5]
During the Warsaw Uprising, Trzeciak tried to shelter a number of refugees in his church. Shortly after, he and several others were shot by German soldiers on 8 August.[4]