Even though he has sinned, he is still a Jew
ישראל אף על פי שחטא ישראל הוא – סנהדרין מד ע”א
“A Jew, even though they have sinned, is still a Jew” – Sanhedrin 44a
On Sunday Cardinal Jean-Marie Lustiger, the former Archbishop of Paris passed away. Born Aaron Lustiger, Cardinal Lustiger converted to Catholicism in 1940 at the age of 13. At the time he was being hidden with his sister from the Nazis. Lustiger did not shy away from discussion of his Jewish roots, he is even reported to have said kaddish on his mother’s yahrzeit. In addition, some of his comments such as, “I was born Jewish and so I remain, even if that is unacceptable for many. For me, the vocation of Israel is bringing light to the goyim. That is my hope and I believe that Christianity is the means for achieving it”, irked numeorus Jewish leaders.
Jacob Katz has shown that the meaning of the Talmudic statement, “A Jew, even though they have sinned, is still a Jew” (Sanhedrin 44a), has changed over time (see his Halakhah ve-Kabbalah, pp. 255-269) . Originally the statement was referring to the people of Israel, even though they may sin, they are still called “Israel.” It was only during the Middle Ages when the phrase took on the halakhic meaning of, “Even though a Jew may sin, even convert, they are still a Jew.” Katz, following Abraham Berliner, attributes the change in meaning to Rashi and places Rashi’s interpretation within the context of conversions, both to and from Judaism, which were occurring during his time.
Cardinal Lustiger’s life brings to mind another similar case, that of Brother Daniel Rufeisen, although Brother Daniel converted as an adult after the war. What made Brother Daniel’s case so famous was that he attempted to acquire Israeli citizenship under the Law of Return (see this contemporary news account). Apparently the Chief Rabbinate supported Brother Daniel’s petition, while with the exception of Judge Haim Cohen, the Supreme Court rejected his petition. One of the judges who voted against Rufeisen was Moshe Silberg who was also a noted scholar of Jewish law. In his decision Silberg wrote the following regarding the statement found in Sanhedrin 44a, “A Jew, even though…”
I will not rely here on the famous statement “A Jew, even though they have sinned…”, since it might truly be (as others have pointed out ) that the level of aggadah is greater than that of halakhah. Irregardless of the nature of the statement, the truth is that the statement functioned as a foothold/stepping stone (?) (מדרס) for halakhah throughout the generations… 
Despite having the Supreme Court reject his petition, Brother Daniel still settled in Israel as a naturalized Israeli citizen, passing away in 1997.
 Silberg is almost definitely referring to Katz since the Supreme Court decision was in 1962 and Katz’s article was originally published in 1958.
 Judge Silberg’s decision is published in his collected writings, Kitvei Moshe Silberg and also in an English translation in Textual Sources for the Study of Judaism although the above translation is mine. Moshe Silberg is also the author of the important book Talmudic Law and the Modern State.