A great set of comments from Bach’s contemporaries.

Bach is like an astronomer who, with the help of ciphers, finds the most wonderful stars”
“Now there is music from which a man can learn something” — W.A. MOZART (on hearing Bach motets in Leipzig)
“Not Brook but Ocean should be his name!” — LUDWIG VAN BEETHOVEN (“Bach” is the German word for “brook”)
“Oh you happy sons of the North who have been reared at the bosom of Bach, how I envy you” — GIUSEPPE VERDI
“Bach is the most stupendous miracle in all music!
“Study Bach! There you will find EVERYTHING!” — JOHANNES BRAHMS
Music owes as much to Bach as religion to its founder — ROBERT SCHUMANN

“And if we look at the works of JS Bach – a benevolent god to which all musicians should offer a prayer to defend themselves against mediocrity – on each page we discover things which we thought were born only yesterday, from delightful arabesques to an overflowing of religious feeling greater than anything we have since discovered. And in his works we will search in vain for anything the least lacking in good taste”

The poetry, the atmosphere, the intensity of expression, the beauty of the preludes and fugues grip, overwhelm, and stimulate us. Let us not be afraid of the supreme contrapuntal science of the fugues, nor be overawed by the stern appearance and heavy wig of Father Bach. Let us gather around him, feel the love, the noble goodness that flow from each one of his phrases and that invigorate and bind us by ties strong and warm.
– Carl Friedrich Zelter (letter to Goethe, 9 June 1827) (Zelter was the teacher of Felix Mendelssohn)

Bach is the supreme genius of music… This man, who knows everything and feels everything, cannot write one note, however unimportant it may appear, which is anything but transcendent. He has reached the heart of every noble thought, and has done it in the most perfect way.
— Pablo Casals

“She played Bach. I do not know the names of the pieces, but I recognized the stiff ceremonial of the frenchified little German courts and the sober, thrifty comfort of the burghers, and the dancing on the village green, the green trees that looked like Christmas trees, and the sunlight on the wide German country, and a tender cosiness; and in my nostrils there was a warm scent of the soil and I was conscious of a sturdy strength that seemed to have its roots deep in mother earth, and of an elemental power that was timeless and had no home in space.”
— W. Somerset Maugham (from ‘The Alien Corn’)

Bach is the beginning and end of all music.
— Max Reger

When eminent biologist and author Lewis Thomas was asked what message he would choose to send from Earth into outer space in the Voyager spacecraft, he answered, “I would send the complete works of Johann Sebastian Bach.” After a pause, he added, “But that would be boasting.”