“NOW I know in part; but then shall I know even as also I am known.”
It is a common practice today to categorize all 19th century composers as part of the “Romantic period”. Certainly, Felix Mendelssohn was also been "pressed" into service. However, Mendelssohn made many unique discoveries that were firmly rooted in the tradition of Bach through Beethoven. Mendelssohn was not of the Romantics camp. He characterized Liszt as having "many fingers, but few brains", and no talent for composition.
Felix Mendelssohn was the grandson of the great philosopher and founder of the Jewish Enlightenment, Moses Mendelssohn, whose salons included such scientific genii as the Humboldt brothers—Alexander was a polymath, geographer, naturalist, and explorer; his older brother Wilhelm was a philosopher and linguist who performed great educational reforms as the the Prussian minister.
Although the Mendelssohn family was steeped in music, Felix and his sister Fanny were way ahead of their time. As a teenager, Felix, with great enthusiasm, would play Beethoven's late String Quartets on the piano for anyone who would listen, even before they were published. Most people found them to be incomprehensible. Even Felix' father Abraham, agreed with the competent, but conservative composer Louis Spohr, that they were: "an indecipherable, uncorrected horror". Spohr represented "the understanding", which respected a fixed set of rules. Beethoven, and the young Mendelssohn, represented "reason", which came, to not overthrow those rules, but to fulfill them.
In 1825, at the age 16, Felix sent his sister Fanny a copy of Beethoven's monumental “Hammerklavier” Piano Sonata No. 29 in B♭ major, Op. 106. While it is often claimed that the first successful performance of this difficult work was by Franz Liszt in 1836, Felix often said, on being praised for his piano playing, "Wait 'til you hear my sister!" She might very well have been the first to play it.
On hearing the news of Beethoven's death in 1827, Felix, then age 18, composed a song, "Frage", or "Question" as part of his (and Fannie's) Op. 9. For a long time the words were credited to a poet named Voss. Later, Felix' nephew insisted that Felix wrote both the music and the poem! It is rare that the poet and composer are the same person! The poem goes as follows:
Frage -Question
Ist es wahr? Ist es wahr?
Daß du stets dort in dem Laubgang,
An der Weinwand meiner harrst?
Und den Mondschein und die Sternlein
Auch nach mir befragst?
Ist es wahr? Sprich!
Was ich fühle, das begreift nur,
Die es mit fühlt,
Und die treu mir ewig,
Treu mir ewig, ewig bleibt.
Question
Is it true? Is it true?
That you will always be there
In the leafy walkway
By the vineyard wall, still waiting?
And that you implore the moonlight
And the stars to tell
Whither have I gone?
Is it true? Pray tell!
What I feel, can only be grasped
By the one whose feels with me
And who true to me ever,
True to me ever, will ever remain.
(Translation by Rick Sanders)
This video includes the words in German only.
Who remained true to whom? Mendelssohn to Beethoven, or Beethoven to Mendelssohn? The short song was an immediate response, in preparation for a much longer work, that took months of work and became the “String Quartet No. 2 in A minor, Op. 13.” In this work, we see a type of love unique to the Classical Principle. Do you know any 18 year olds who show such emotional maturity? This is not sensuous love. It is not grandfatherly love. It is the love of creativity, the love of one creative mind for another, not for its own sake, but for sake humanity! Mendelssohn saw himself in Beethoven, found himself in Beethoven, and pledged to remain true to Beethoven. And, Beethoven found himself in Mendelssohn-remained true to Mendelssohn.
But how could Beethoven, who could not possibly have known of Felix' existence, remain true to him?
“WHAT I feel is only understood by the one who feels it with me." 
When Felix played those late quartets for family and friends, he may have been the only one (outside of Schubert and a few others in Vienna) who deeply felt the creativity that went into them. When Felix composed, his conscience would have envisioned Beethoven looking over his shoulder, gently.
Beethoven was one of the most uncompromising minds in history. He never compromised on principle. When other artists degenerated morally during the intellectual repression that followed the Congress of Vienna in 1812, Beethoven demanded new levels of creativity from himself, and was adamant that he was not doing it for his contemporaries, but for the future. Beethoven was faithful to humanity, to his Creator, to himself, and especially, to the future (including an unknown Felix in the cradle.)
“BUT then shall I know even as also I am known.”
The first movement of Mendelssohn's “Quartet in A Minor” starts out with a slow theme that is reverential and Beethovenesque. A couple of minutes in, Mendelssohn plays the three-note theme of “Ist es war?” (Is it true?), from the song “Frage”. Compare it to Beethoven's three-note motiv, “Muss es sein?” (must it be?) from the Fourth movement of his “String Quartet No. 16 in F Major, Op. 135.” (please listen to the audio file: https://drive.google.com/.../10A2rjiv41tz8.../view....)
So, what does "Ist es war" mean? In the most simple sense, is it true that Beethoven is dead? For that matter, what does Beethoven's "Muss es sein" mean? Simply, "Must I die?" (Op 135 was Beethoven's last completed work). Or, do they both mean more?
Mendelssohn's quartet is in A Minor, and bears an overall relation to Beethoven's String Quartet No. 15, Op. 132, also in A Minor. The audio compares the opening of the first movement (after a short introduction), of Beethoven's Op. 132, with the opening of the first movement of Mendelssohn's Op. 13 (again after a short intro). The connection is hard to miss.
Then we present the recitative-like section that is the transition from movement 4 to movement 5 in Beethoven's Op. 132, with the recitative-like section, that is the transition from movement 3 to movement 4 in Mendelssohn's Op. 13.
Then we compare the fughetta from the second movement of Beethoven's String Quartet No. 11 in F minor, Op. 95, with the fughetta from the second movement of Mendelssohn's Op. 13.
These quotes are loving and unmistakable, from three late Beethoven quartets, some of which had not even at the time, yet been published (Op. 95, Op. 132, and Op. 135)!
The fourth movement of Op. 13 begins quickly, but suddenly halts, and ends the quartet with an incredible synopsis of the entire process.
Here is the all-important audio file again:
https://drive.google.com/.../10A2rjiv41tz8.../view...
If music history valued creativity and agape, the highest form of love, this work would be much more in the center of the repertoire than it currently is. We provide the entire quartet with score, for those whose curiosity is sufficiently piqued!