In 1941, while serving as a pastor in Lübeck, he met Johannes Prassek, a Catholic priest from the nearby Sacred Heart Catholic Church (). They shared their disapproval of the Nazi regime, and Prassek introduced Stellbrink to his Catholic colleagues, Frs Hermann Lange and Eduard Mueller. The four priests spoke publicly against the Nazis – initially discreetly – distributing pamphlets to friends and congregants. They copied and distributed the anti-Nazi sermons of the Catholic Bishop Clemens August von Galen of Münster. Then, following the RAF air raid on the city, after which Stellbrink tended wounded, he delivered a Palm Sunday sermon which attributed the bombing to divine punishment. Stellbrink was arrested, followed by the three Catholic priests. He was sentenced to death by the People’s Court on 23 June 1943, and executed in Hamburg on 10 November 1943, along with the three Catholics. The mingling of the blood of the four guillotined martyrs has become a symbol of German Ecumenism.After his death, Stellbrink's widow was billed for his court costs, imprisonment, and execution.