I do not think his parents were German Jews, but Polish from Bedzin. Any Jewish convert to Catholicism, whether it was the late Cardinal Arachbishop, Edith Stein, Max Jacob,Henri Bergson, Fr. Elias Freidman, Rina Geftman, Brother Daniel, all of them, in their mind, remained Jews, "fulfilled" Jews as they would term themselves. Of course the former Chief Rabbi would find this to be a gross contradiction, but for the converts themselves, it needed no explanation. Lustiger could say Kaddish for his mother and pray for her at Mass as well. What few if any of these converts can say, regardless of how they perceive of themselves is, and my offspring are Jewish as well. That even they cannot say, though they might think of themselves a fulfilled Jews until they enter into eternal life.
Palestinians, IDF clash in Jalazone after funeral of teen killed by IDF on Friday (Haaretz)
from the article: 'He'd say kaddish for his mother'