"Sie wird das nothwendigste und härteste und die hauptsache in der Musique niemahlen bekommen, nämlich das tempo, weil sie sich vom jugend auf völlig befliessen hat, nicht auf den tact zu spiellen."
She will never learn the most necessary, most difficult and principal thing in music, that is time, because from childhood she has designedly cultivated the habit of ignoring the beat.
Letter to Leopold Mozart (24 October 1777), from Mozart: The Man and the Artist, as Revealed in his own Words by Friedrich Kerst, trans. Henry Edward Krehbiel (1906) 
I know myself, and I have such a sense of religion that I shall never do anything which I would not do before the whole world; but I am alarmed at the very thoughts of being in the society of people, during my journey, whose mode of thinking is so entirely different from mine (and from that of all good people). But of course they must do as they please. I have no heart to travel with them, nor could I enjoy one pleasant hour, nor know what to talk about; for, in short, I have no great confidence in them. Friends who have no religion cannot be long our friends.
Letter to Leopold Mozart (Mannheim, 2 February 1778), from The letters of Wolfgang Amadeus Mozart, 1769-1791, translated, from the collection of Ludwig Nohl, by Lady [Grace] Wallace (Oxford University Press, 1865, digitized 2006) vol. I, # 91 (p. 164) 
The most stimulating and encouraging thought is that you, dearest father, and my dear sister, are well, that I am an honest German, and that if I am not always permitted to talk I can think what I please; but that is all.
Letter to Leopold Mozart (Paris, 29 April 1778), from Mozart: The Man and the Artist, as Revealed in his own Words by Friedrich Kerst, trans. Henry Edward Krehbiel (1906)
I must give you a piece of intelligence that you perhaps already know — namely, that the ungodly arch-villain Voltaire has died miserably like a dog — just like a brute. That is his reward!
Letter to Leopold Mozart (3 July 1778), from The letters of Wolfgang Amadeus Mozart, 1769-1791, translated, from the collection of Ludwig Nohl, by Lady [Grace] Wallace (Oxford University Press, 1865, digitized 2006) vol. I, # 107 (p. 218) 
A fellow of mediocre talent will remain a mediocrity, whether he travels or not; but one of superior talent (which without impiety I cannot deny that I possess) will go to seed if he always remains in the same place.
Letter to Leopold Mozart (11 September 1778), from Wolfgang Amadé Mozart by Georg Knepler (1991), trans. J. Bradford Robinson [Cambridge University Press, 1994, ISBN 0-521-58823-5], p. 12.
Letter to Leopold Mozart, (Mannheim, 12 November 1778), from Mozart's Letters, Mozart's Life: Selected Letters, ed. Robert Spaethling [W.W. Norton, 2000, ISBN 0-393-04719-9], p. 193.
My fatherland has always the first claim on me.
Letter to Leopold Mozart (24 November 1781), from Mozart: The Man and the Artist, as Revealed in his own Words by Friedrich Kerst, trans. Henry Edward Krehbiel (1906).
The golden mean, the truth, is no longer recognized or valued. To win applause one must write stuff so simple that a coachman might sing it, or so incomprehensible that it pleases simply because no sensible man can comprehend it.
in a letter to his father, 1782
"Will mich Deutschland, mein geliebtes Vaterland, worauf ich (wie Sie wissen) stolz bin, nicht aufnehmen, so muß in Gottes Namen Frankreich oder England wieder um einen geschickten Deutschen mehr reich werden,- und das zur Schande der deutschen Nation."
If Germany, my beloved fatherland, of whom you know I am proud, will not accept me, then must I, in the name of God, again make France or England richer by one capable German; — and to the shame of the German nation.
Letter to Leopold Mozart (Vienna, 17 August 1782), from Mozart: The Man and the Artist, as Revealed in his own Words by Friedrich Kerst, trans. Henry Edward Krehbiel (1906).
I care very little for Salzburg and not at all for the archbishop: I shit on both of them.
Letter to Leopold Mozart (12 July 1783), from The Complete Operas of Mozart: a critical guide By Charles Osborne [Da Capo, 1983, ISBN 0-306-80190-6], p. 208.
As death, when we come to consider it closely, is the true goal of our existence, I have formed during the last few years such close relationships with this best and truest friend of mankind that death's image is not only no longer terrifying to me, but is indeed very soothing and consoling, and I thank my God for graciously granting me the opportunity...of learning that death is the key which unlocks the door to our true happiness. I never lie down at night without reflecting that —- young as I am — I may not live to see another day. Yet no one of all my acquaintances could say that in company I am morose or disgruntled.
Letter to Leopold Mozart (4 April 1787), from The Mozart-Da Ponte Operas by Andrew Steptoe [Oxford University Press, 1988, ISBN 0-198-16221-9], p. 84.
It is a mistake to think that the practice of my art has become easy to me. I assure you, dear friend, no one has given so much care to the study of composition as I. There is scarcely a famous master in music whose works I have not frequently and diligently studied.
Spoken in Prague, 1787, to conductor Kucharz, who led the rehearsals for Don Giovanni, from Mozart: The Man and the Artist, as Revealed in his own Words by Friedrich Kerst, trans. Henry Edward Krehbiel (1906).
Melody is the essence of music. I compare a good melodist to a fine racer, and counterpointists to hack post-horses; therefore be advised, let well alone and remember the old Italian proverb: Chi sa più, meno sa—Who knows most, knows least.
As spoken to Michael Kelly, from Reminiscences of Michael Kelly, of the King's Theatre, and Theatre Royal Drury Lane, including a period of nearly half a century; with Original Anecdotes of many distinguished Personnages, Political, Literary, and Musical (London, Henry Colburn, 1826; digitized 2006), 2nd ed., vol. I (p. 225) 
Stay with me to-night; you must see me die. I have long had the taste of death on my tongue, I smell death, and who will stand by my Constanze, if you do not stay?
Spoken on his deathbed to his sister-in-law, Sophie Weber (5 December 1791), from Mozart: The Man and the Artist, as Revealed in his own Words by Friedrich Kerst, trans. Henry Edward Krehbiel (1906)
Variant: The taste of death is on my tongue, I feel something that is not from this world (Der Geschmack des Todes ist auf meiner Zunge, ich fühle etwas, das nicht von dieser Welt ist).
The passions, whether violent or not, should never be so expressed as to reach the point of causing disgust; and music, even in situations of the greatest horror, should never be painful to the ear but should flatter and charm it, and thereby always remain music.
Letter by Mozart, as quoted in a journal entry (12 December 1856) The Journal of Eugene Delacroix as translated by Walter Pach (1937), p. 521.
All I insist on, and nothing else, is that you should show the whole world that you are not afraid. Be silent, if you choose; but when it is necessary, speak—and speak in such a way that people will remember it.
Letter as published in The Letters of Mozart & His Family (1938) translated and edited by Emily Anderson, p. 1114.
Neither a lofty degree of intelligence nor imagination nor both together go to the making of genius. Love, love, love, that is the soul of genius.
True genius without heart is a thing of nought - for not great understanding alone, not intelligence alone, nor both together, make genius. Love! Love! Love! that is the soul of genius. - Nikolaus Joseph von Jacquin, entry in Mozart's souvenir album (1787-04-11) from Mozart: A Life by Maynard Solomon [Harper-Collins, 1966, ISBN 0-060-92692-9], p. 312.
When I am, as it were, completely myself, entirely alone, and of good cheer — say traveling in a carriage, or walking after a good meal, or during the night when I cannot sleep — it is on such occasions that my ideas flow best, and most abundantly. Whence and how they come, I know not, nor can I force them.
From a letter now regarded as a forgery by Johann Friedrich Rochlitz , , 
I pay no attention whatever to anybody's praise or blame. I simply follow my own feelings.
Unsourced in Musician's Little Book of Wisdom (1996) by Scott E. Power, Quote 416.
It may be that when the angels go about their task praising God, they play only Bach. I am sure, however, that when they are together en famille they play Mozart.
Wolfgang Amadeus Mozart (1956), as translated by Clarence K. Pott
Mozart's music is free of all exaggeration, of all sharp breaks and contradictions. The sun shines but does not blind, does not burn or consume. Heaven arches over the earth, but it does not weigh it down, it does not crush or devour it. Hence earth remains earth, with no need to maintain itself in a titanic revolt against heaven. Granted, darkness, chaos, death and hell do appear, but not for a moment are they allowed to prevail. Knowing all, Mozart creates music from a mysterious center, and so knows the limits to the right and the left, above and below. He maintains moderation.
As quoted in "Amadeus : Sin, Salvation and Salieri" in Saint Paul at the Movies : The Apostle's Dialogue with American Culture (1993) by Robert Jewett, p. 37, and Melting the Venusberg : A Feminist Theology of Music (2004) by Heidi Epstein, p. 72.
21 piano sonatas, 27 piano concertos, 41 symphonies, 18 masses, 13 operas, 9 oratorios and cantata, 2 ballets, 40 plus concertos for various instruments, string quartets, trios and quintets, violin and piano duets piano quartets, and the songs. This astounding output includes hardly one work less than a masterpiece.
Lengthy immersion in the works of other composers can tire. The music of Mozart does not tire, and this is one of its miracles.
Listening to Mozart, we cannot think of any possible improvement.
Around the beginning of our century, we find the "rediscovery" of Mozart, which has only been intensified through the propagation of chaotic music and the obvious necessity which follows to return to real beauty and greatness in music. ... I can remember when one of his symphonies was used to fill up an empty space in a programme where the main dishes were Beethoven, Wagner, etc. He was thought of as a trinket, charming, delicious, yes—but a trinket all the same.
Mozart is undoubtedly one of the greatest of original geniuses, and I have never known any other composer to possess such an amazing wealth of ideas. I wish he were not so spendthrift with them. He does not give the listener time to catch his breath, for no sooner is one inclined to reflect upon a beautiful inspiration than another appears, even more splendid, which drives away the first, and this continues on and on, so that in the end one is unable to retain any of these beauties in the memory.
Karl Ditters von Dittersdorf, in his Autobiography
Mozart's influence transcends history. Each generation sees something different in his work Mozart's music, which to so many of his contemporaries still seemed to have the brittleness of clay, has long since been transformed into gold gleaming in the light, though it has taken on the different luster of each new generation No earthly remains of Mozart survived save a few wretched portraits, no two of which are alike; the fact that all the reproductions of his death-mask, which would have shown him as he really was, have crumbled to bits seems symbolic. It is as though the world-spirit wished to show that here is pure sound, conforming to a weightless cosmos, triumphant over all chaotic earthliness, spirit of the world-spirit.
Alfred Einstein, Mozart: His Character, His Work
I tell you before God and as an honest man that your son is the greatest composer known to me; he has taste and in addition the most complete knowledge of composition.
Franz Joseph Haydn, to Mozart's father, Leopold, after hearing the six quartets Mozart dedicated to him in 1785
You wish me to write an opera buffa for you. Most willingly, if you are inclined to have a vocal composition of mine for yourself alone, but if with a view to produce it on the stage at Prague, I cannot in that case comply with your wish, all my operas being too closely connected with our personal circle, so they could never produce the proper effect. ... But even then I should risk a great deal, for scarcely any man could stand beside the great Mozart.
I only wish I could impress on every friend of mine, and on great men in particular, the same depth of musical sympathy and profound appreciation of Mozart's inimitable music that I myself feel and enjoy; then nations would vie with each other to possess such a jewel within their frontiers.... It enrages me to think that the unparalleled Mozart is not yet engaged by some imperial or royal court! Forgive my excitement, but I love the man so dearly!
Franz Joseph Haydn, in a letter to Herr Roth, 1787
There is a wretched unbelief abroad which seems to contain much healing power. It deems such a connection accidental, and sees in it only a lucky conjunction of the different forces in the game of life. It thinks it an accident that the lovers win one another, accidental that they love one another; there are a hundred other women with whom the hero would have been equally happy, and whom he could have loved as deeply. It thinks that there has been many a poet who might have become as immortal as Homer, if this splendid subject had not already been appropriated by him; many a composer who might have made himself as immortal as Mozart, had the opportunity offered. … The accidental has but one factor; it is accidental that Homer found in the Trojan War the most distinguished epic subject conceivable. The fortunate has two factors: it is fortunate that the most distinguished epic subject fell to the lot of Homer; here the accent falls as much on Homer as on the material. It is this profound harmony which reverberates through every work of art we call classic. And so it is with Mozart; it is fortunate that the subject, which is perhaps the only strictly musical subject, in the deeper sense, that life affords, fell to — Mozart.
Your countenance ... was so grave that many intelligent persons, seeing your talent so early developed and your face always serious and thoughtful, were concerned for the length of your life.
Leopold Mozart, in a letter to his son
But what is it about Mozart? Is there a pianist alive who really manages to play him well? Casadesus, whom I heard in Odessa in the F major Sonata K 332 – it must be about a century ago – left an unforgettable impression, a miracle such as one rarely witnesses. And then there was Neuhaus, who played the A minor Rondo in so touching a manner that it almost reduced you to tears. It's odd, but Haydn – who seems after all to be fairly close to Mozart in terms of genius – is infinitely less difficult to play (he's almost easy in fact).
So what's Mozart's secret?
Sviatoslav Richter, in Bruno Monsaingeon, Sviatoslav Richter: Notebooks and Conversations (2001)
Pascal and Felix Mendelssohn were prodigiously precocious. But when each died before reaching age forty, each was physiologically an old man. Not so with Mozart – from him could have been extrapolated as much again in the future as had generously erupted in the past.
What was evident was that Mozart was simply written down music already finished in his head. And music, finished as no music is ever finished. Displace one note and there would be diminishment. Displace one phrase and the structure would fall. I was staring through the cage of those meticulous ink strokes at an absolute beauty.
Mozart was the Shakspeare of music; and as long as the immortal bard is read, Mozart will live in the admiration of mankind. He has reached the passions through the ear as Shakspeare did through the mind, and no works will live that do not touch the passions and the heart — they are the same in all ages, and will make Shakspeare and Mozart a poet and a composer "For all time".
The New-York Mirror, 1830, quoted in The New York Mirror: A Weekly Gazette of Literature and the Fine Arts, volume 8, p. 347.
In relation to God he is like a child who brings everything to his father: the stones from the street and peculiar sticks and little plants and even once a ladybug; and with him all of these things are melodies, melodies that he brings to God, melodies that he suddenly knows when he is inside of prayer. And when he has finished praying, and he is no longer on his knees and no longer has his hands folded, then he sits there at the piano, or he sings with an incredible childlikeness, and in doing so he no longer has any idea whether he is playing something for God or whether it is God who is using him to play something at once for himself and for Mozart. There is a great conversation between Mozart and God that is the purest prayer, and this entire conversation is nothing but music.