There are two major points to keep in mind if you want to understand the real significance of Auerbach.
First, as David Damrosch has noted in the best essay I've ever read about Auerbach, Mimesis is an assertion, in the face of the Nazi demolition of culture, of the enormous humanistic, and humane, and simply human, value of philology. Damrosch:
Begun in exile in 1942 and completed in April 1945 (the very month of Hitler's death), Mimesis stands as an affirmation of the scholar's ability to rise above every obstacle that adverse historical circumstances can present. Or, to put it differently: Auerbach responds to the loss of his homeland and the collapse of his scholarly world through the recreation of European culture, both in the evocation of texts from across the tradition and by the display of humanistic scholarship at its best, with analyses at once judicious and loving, objective and deeply personal.
Philology is for Auerbach (as it was in different ways for Nietzsche, Tolkien, and A. E. Housman) the humanistic discipline par excellence, the intellectual nexus where deep historical learning and the mastery of multiple languages converge with refined taste and sensibility. It is everything Nazism is not. It is the last bastion of the Republic of Letters, where neither nationality nor ethnicity mean anything, only a passion for learning and a love of the beautiful.
That is the first point. The second is this: Mimesis is a document of hope, hope grounded in what Mikhail Bakhtin called “great time.” Great time concerns history as it unfolds over, not decades, not even centuries, but millennia. Bakhtin believed there are meanings implicit in, say, Athenian tragedy that will only be discovered in the distant future. As he wrote in a notebook near the end of his life,
There is neither a first nor last word and there are no limits to the dialogic context (it extends into the boundless past and the boundless future). Even past meanings, that is, those born in the dialogue of past centuries, can never be stable (finalized, ended once and for all) — they will always change (be renewed) in the process of subsequent, future development of the dialogue. At any moment in the development of the dialogue there are immense, boundless masses of forgotten contextual meanings, but at certain moments of the dialogue’s subsequent development along the way they are recalled and reinvigorated in renewed form (in a new context). Nothing is absolutely dead: every meaning will have its homecoming festival.
This is very much the implicit argument of Mimesis: in his brilliant chapter on the Gospel narratives, for instance, Auerbach shows how those stories upend the classical model of stylistic decorum — a high style for high things, a low style for low things — in ways that would not bear ripe literary fruit for nearly eighteen hundred years.
So even if the Nazis were winning as Auerbach wrote — in his Istanbul exile — they would not win forever. They would be unable to eradicate the great Western culture that Auerbach had devoted his life to; it would return, it would find new life, its deepest and richest meanings would eventually have their homecoming festival. In its hopefulness Mimesis is a passionate act of defiance, the defiance of the truly cultured in the face of culture's powerful defilers.
That's what Mimesis is all about, and that's why it's one of the greatest books of the twentieth century.