It is interesting that the facts remain the same – the date of conversion, the family relationships surrounding Mendelssohn – but the interpretation of his feelings, his own identity, and his own actual beliefs are easily swayed one way or another. Since Mendelssohn himself wrote little about his beliefs, researchers can only infer from his actions, his music, and small hints here and there, such as the keeping of his grandfather's family name.
An article by Manfred Lehmann, of the Lehmann Foundation: http://www.manfredlehmann.com/sieg236.html
"In Felix's voluminous letters there is hardly a mention of Christianity. Felix was a Jew until 1816 when he was baptized at the age of 7, while his parents remained Jews until 1822 when they, too, were baptized. Felix was almost bar mitzvah age in that year."
"Besides his family, Felix Mendelssohn also socialized with the well-known Jewish banker, Salomon Heine, the uncle of the famous bard, Heinrich (Chayim) Heine—also a convert of convenience—who retained his Jewish consciousness very conspicuously throughout his poems. (When a childhood friend once asked Heine if he really believed in Jesus, he answered: "Have you ever met a Jew who has faith in another Jew?")"