The innocent reason inquiring about Jewishness in this context makes sense is because a substantial number of self-identified Jews are also self-described atheists or agnostics. People who identify as Christian are virtually all theists. If they come from an ancestrally Christian tradition but are atheists or agnostics by belief, they identify as having no religion. Many Jews do not do this.
The Muslim tendency in the US appears* to fall in between that of Jews and Christians. The following table shows the percentages of self-identified Protestants, Catholics, Jews, and Muslims who also self-identify as atheistic or agnostic:
GSS variables used: YEAR(1998-2014), RELIG(1,2,3,9), GOD(1-2)
* The small Muslim sample is worth noting. Sample sizes are 6913, 3329, 231, and 65 for Protestants, Catholics, Jews, and Muslims, respectively.