Let's start by rewriting the quote in modern English so we can fully understand its meaning:
You can’t always judge a book by its cover. People are often tricked by false appearances.
High modality could apply to this quote, particularly through the use of the strong/forceful "be least" and "is still" to strengthen the author's idea/the character's perspective of the subject they are addressing within the text.
I think there might also be some sort of
motif associated with this quote as well as (potentially) the whole text. Suggesting that one cannot make an informed judgement by solely examining the "outward" and the fact that people are "deceived with ornament" may create/contribute to the creation of a motif (such as an idea, symbol or theme) which will be present throughout the entire text. I'm not familiar with this text though, so you might want to check how applicable this is based on your own knowledge of the text.
I hope this helps!