Bernstein records Mendelssohn’s career without commenting on his personal faith, or the Jewish and Christian aspects of his musical inspiration and output – these were well enough known and a matter of discussion in 1909, when “Some Jewish Witnesses for Christ” was published:
Mendelssohn, Felix Bartholdi, was born at Hamburg, February 3, 1809, and died 1857. When he was four years old his parents removed to Berlin. His father at once procured teachers in music for him, as he had begun thus early to show great talent in that direction. His teachers on the piano were Louis Bezer, and Zelter, the friend of Goethe. The chapel choirmaster, Mr. Hennings, gave him instruction on the violin. The father of the poet, Paul Heyse, who later became the celebrated philologist, was his private tutor in the home of the Mendelssohns, where the intellectual aristocracy of Berlin frequently assembled.
When Felix was nine years old he appeared for the first time at a public concert, where his remarkable performances won him great applause. Two years later, we find him in a Berlin Musical Academy, where he studied church music under Zelter’s direction. When he was twelve years old, Zelter placed him in the Olympic in Weimar, where he made remarkable progress. When he was  fourteen, we find him a guest at Goethe’s house, and his host wrote thus to Zelter:—”Felix’s productions astonish everybody.” No one was more delighted at the boy’s success than his father, who took pride in gratifying his son’s every wish regarding his musical education, and the latter’s diligence amply rewarded any outlay.
Before Felix was out of his teens he had written four operas. His father accompanied him to Paris, where he had the education of the best teachers of the time. Soon after he went to London, where he wrote an original overture founded upon Shakespere’s “Midsummer Night’s Dream,” which attracted the attention of the celebrated singer, Henrietta Sonntag, and won a great triumph for its composer—he was then twenty years old. He became a member of a Philharmonic Society. He spent some time in Rome, where he composed “Die Walpinges nacht,” and arranged the one hundred and fifteenth psalm to music. He also visited Naples. This Italian town made a lasting impression upon his mind. He played before many of the crowned heads of Europe. King Frederick William IV. of Prussia was greatly interested in the young composer, and employed him to write the music for the “Tragedy of Sophocles.” His success greatly excited the jealousy of the older musicians, but the King became his dearest friend.
Mendelssohn played in 1841 before Queen Victoria. He thus described the occasion in a letter to his “dear little mother.” “I asked Prince Albert, the Queen’s husband, to play something on the organ for me. He complied. His playing—so beautiful  and perfect—many an organist might have envied him. Then I played and sang my chorus from “St. Paul,” “How beautiful are the messengers.” When I had finished the first stanza, the Queen and Prince Albert joined in the singing. The Queen asked if I had any new compositions; if so, she would gladly have them printed. We went into her salon, where there was a piano. I played and sang again. She praised my playing and singing, and when I bade them adieu said: ‘I hope you will soon visit us in England again.'”
This brilliant career was speedily cut short. The death of his dearly beloved sister Fanny, in the spring of 1847, affected him seriously. All his compositions thereafter were melancholy. He became nervous and irritable. He could not apply himself to his work, but would sit for hours with his hands folded. After a brief illness he died on November 4, 1857, when he was only thirty-six. Three days after, he was carried to his grave by the side of his sister Fanny Hensel, in Trinity Cemetery, Berlin.
Felix Mendelssohn was a favourite of the German people—a musical genius like Weber and Schubert. He put his whole life and soul into his work. His early death confirms Neander’s words—also a Hebrew Christian—”Whom the gods love die young.” God gave him a musical gift, which he delighted to use for His glory.
For Mendlssohn’s personal faith, Jewish influence in his music, encounter with anti-Semitism (particularly in Wagner’s opposition to him as a composer, Jew and rival), see here. Mendelssohn did not discuss his own personal faith, but expressed great religious themes in his compositions. Grandson of the Jewish philosopher Moses Mendelssohn, he always affirmed his Jewish identity. Yet he was baptised at the age of seven, on 21 March 1816. Deborah Hertz discusses the phenomenon of assimilation here
Hear my prayer, O God, incline Thine ear!
Thyself from my petition do not hide.
Take heed to me! Hear how in prayer I mourn to Thee,
Without Thee all is dark, I have no guide.
The enemy shouteth, The godless come fast!
Iniquity, hatred, upon me they cast!
The wicked oppress me, Ah where shall I fly?
Perplexed and bewildered, O God, hear my cry!
My heart is sorely pained, within my breast,
my soul with deathly terror is oppressed,
trembling and fearfulness upon me fall,
with horror overwhelmed, Lord, hear me call,
O for the wings, for the wings of a dove!
Far away, far away would I rove!
In the wilderness build me a nest,
and remain there for ever at rest.
Hear my prayer, F Mendelssohn Bartholdy