Norwegian Jew?

My g-grandfather & g-grandmother were born in Norway, according to my grandfathers birthcertificate. A now deceased relative confided in my mother (years ago) that our family name was completely different than what we now know (Johnson) it to be. She told my mother the name, but Mom can't remember it. She does remember my aunt telling her that we could be Jews. They immigrated to the US and settled in Philadelphia. The birthcertificate I have lists g-grandfather as Charles B Johnson (born abt 1871-1872. His occupation is listed as "rigger". It lists g-grandmother's maiden name as Albertina Miller (born abt 1874-1875). I believe their first child was born in the US, which means they couldn't have arrived here later than 1886. I feel certain that the names Charles and Albertina are true. Can someone point me in the right direction?