Is Yiddish Jewish?

My family is listed in the 1930s Census as being Yiddish. They were from Lithuania. I have always said I was biologically Jewish, but not religiously. Though they were listed as Yiddish, they weren't raised with much religion, but celebrated Christmas and Easter. Though we do have certain stereotypical Jewish behavior in some cases, such as personality and movements. So when it said Yiddish on the Census, are we Jewish or what did they mean?