A fairly high percentage of non-Orthodox Jews are actually goyim with Jewish ancestors. While there might have gotten them in trouble with the Nazis, from our perspective they are 100% goyim. Since it has been over two centuries since the non-Torah movements in Judaism began, we should be dubious of the yichus (meaning in this case, “Jewishness”) of any non-Orthodox Jews and regard them as safek Jews (meaning that while we can’t ask them to do work for us on Shabbos, we don’t want them touching non-mevulshal wine, etc.; while we should feel obligatged to bury them, we wouldn’t want our kids to marry their kids). There are also shailohs since many hold that their marriages were valid, but their divorces weren’t, and children of remarriages are mamzerim.
Of course if they do Tseuvah, being safek goy, safek mamzer is not so bad, since they can quickly and easily convert and rejoin klal Yisrael.