Hidden Jewsfor Jews to hide in Christian society was a daunting task. Some of the challenges have been discussed by Nechama Tec with regard to Poland, and are applicable with modifications to other countries:
Access-reaching the Christian world required an illegal departure from a ghetto, a transport or a camp;
Abandonment-hiding often meant breaking up a family unit;
Documents-passing required several types of official documents or forgeries;
Familiarity with the Christian religion-a new identity required being familiar with Christian religious customs of which Jews had no knowledge;
Appearance-Jews with the physical characteristics of curly black hair, dark eyes, dark complexion, a long nose were in special jeopardy;
Circumcision-Jewish males were invariably circumcised while Polish and many European Christians were not, this provided a certain method of identifying Jewish men;
Language-most Jews spoke Polish with an accent or used expressions derived from Yiddish which gave them away;
Help-hiding often meant relying on Christian rescuers who were willing to risk death or being sent to a concentration camp;
Money-money or items of value were needed to pay those rescuers who required payment, to purchase food on the black market (hidden Jews did not have ration books), to purchase counterfeit documents, and to pay bribes to blackmailers;
Enemies-Germans, Polish police, even Jewish police and blackmailers were a threat. Some Poles practiced blackmail as an occupation, the activity of blackmail was known as Szmalcownik (from the Polish word "szmalec" which means "fat").
Source: Tec, When Light Pierced the Darkness, pgs. 27-51.