> Mendelssohn or Heine,
even in these two examples – I don’t think either was a kofer. M. was a personally observant Yid who was looking for an honest response to modernity, having disadvantage being one of the first to be confronted with it. His efforts were mostly a failure, so what. R Yochanan till the end of his life was not sure whether his conversation w/ Vespasian was or was not a failure. Many learned people failed in their anticipation and response to Russian revolution and Nazi occupations, it does not diminish their achievements. We are not Neviim.
Heine converted out of social considerations after being raised in non-religious environment, not any religious feelings. He is clear about that. He was coming back to his Jewishness later on. See his poem about Ibn Gabirol and his later diaries.