One finding of the Pew study that was curious to me is that there seem to be a higher % of secular Americans than secular Jews. I think there is good reason to believe that there are far more practicing secular Jews than even the Pew Study lists. The issue is a matter of how responses were coded.
One stat that has been fascinating me is the answers to the “how important is religion to your life?” question. For the Orthodox that number is 83% “very” increasing to 98 when you include “somewhat” as well. For the Conservative the number of “very important” is 43% and Reform is 17%. In fact, the vast majority of Reform Jews list religion as “not at all” important. (I’ve written an article for the Jewish Light this week that addresses the implications of this finding to how we think about the relative challenges for the movements.)
My guess is that anyone who merely identifies by denomination but for whom religion is “not at all” important are in practice secular or non-religious Jews even if they define their Jewishness in terms of religion. If that’s so, the number of “non-religious” Jews is considerably higher than categorized and more in keeping with the national trend.
Why would someone say they were a Jew by religion if religion were not important to them? Because for Americans –Jews and non-Jews–the category “Jew” is seemingly a religious category. Thus a person who identifies as a Jew may well say, “yes, my religion is Jewish (for what else does it mean to be a Jew?) but religion is not particularly important to me.” By contrast the Christian faiths are less susceptible to that coding issue. If a person says “I am a Protestant” or “I am a Catholic” religion is likely to be somewhat important to them or they would not identify as such. Indeed, many Americans who reported “none” for their religious affiliation may have said “I used to be Methodist (etc.).”
For Jews, the category “used to be Jewish” is not usually seen as viable. Thus, if one is going to count oneself as a Jew they might opt for “Jew by religion” even if religion is not important and they are in practice secular.