Someone's Jewishness is the fact that they are a Jew.
Jewishness in American English
(ˈdʒuːɪʃnɪs)
noun
the state or quality of being Jewish
Word origin
[1540–50; jewish + -ness]This word is first recorded in the period 1540–50. Other words that entered Englishat around the same time include: gondola, labyrinth, mandate, platform, telltale-ness is a suffix attached to adjectives and participles, forming abstract nouns denotingquality and state. Other words that use the affix -ness include: carsickness, consciousness, nothingness, responsiveness, rockiness