For many years I believed that there was a little information available for the modern researcher describing the situation of the Jewish believers (Messianic Jews, Hebrew Christians etc.) within the Warsaw Ghetto. The following brief and profound statement by Rabbi Dan Cohen Sherbock in his book entitled Messianic Judaism sums up my research findings as well,

Following the Holocaust, missions to the Jewish people ceased to function in those cities where the Jewish population had been largely eliminated. In Warsaw, for example, only a few hundred Jews survived the war; most of the workers stationed there escaped before the outbreak of the war, while others died in the Warsaw Ghetto or in one of several Concentration camps. ...