An important thing for allies of the Jewish community to understand:
I commonly see people referring to Judaism as a 'religion' through the same lens that they view Christianity, which is a universal religion, this is a mistake.
Judaism is a semi-closed ethno-religion, it's not simply a religion where only the adherents are called Jews.
Judaism is the religion of the Jewish people; a people who are united by ethnicity, language, culture, cuisine, tradition, shared history, ancestral heritage and so much more, religion is only one facet of it.
To be religiously Jewish you must already be a Jew, either by birth or by conversion (which is a long and difficult process, after which you are counted as ethnically Jewish).
Secular Jews who are not religious in any way are just as Jewish as the most devout Jews.