You had it right, it is a simile because it uses the word as.
If you're referring to the one who quote that sentence, it would be unknown, but it is a really famous quote.
If
you wanted to restates the main idea of this quotation , it would be :
it's not good to criticize others because we don't know that they face
in life.
hope this helps!
Answer:
Change now to "at the moment".
Explanation:
then or at the moment are good changes of now into reported speech.