She would presume to censure my own conduct, and press on me advice in matters well beyond her understanding, and give me such words as no poor man would willingly hear from his wife.’
Cranmer says, ‘She was bold, it is true. She knew it for a fault and would try to bridle herself.’
Bring Up the Bodies by Hilary Mantel
What does the bold sentence mean? She knew it to be a fault?
It means exactly the same as "She knew that it was a fault".
It's an old-fashioned, literary phrasing and you would almost never hear it in conversation today.
FROM OXFORD ENGLISH DICTIONARY -
a. In the character of, in the light of, as equivalent to; esp. to introduce the complement after verbs of incomplete predication, e.g. to have, hold, etc. (see those verbs), where as or as being may generally be substituted. to beg (a person) for a fool.
This is the meaning OED gives.
Now your sentence is -
She knew it for a fault.
From the dictionary it is equivalent to a fault. Or this sentence can be rephrased as -
She knew it to be a fault.
