That's a very vague and philosophical question.
But for purposes of a person born outside of Israel, Israeli citizenship has no requirement that a person believe in God, the Old Testament, Purim, dietary rules, old myths, speak or read Hebrew, have any philosophy on religion, or even understand Israeli law (unlike the US citizenship test which has constitutional questions). The test is solely based upon either the matriarchal history or an Orthodox conversion.
In short, there is NO BELIEF component for those born to a Jewish mother. Conversions are a different animal - I understand that to become a member of the tribe, newcomers must satisfy the Orthodox branch of Judaism which does have a serious belief component.