The consciousness of acting right diffused a serenity over his manners, which nothing else could impart to a man of moral perceptions like his, and which refined his sense of every surrounding blessing.
THE MYSTERIES OF UDOLPHO by Ann Radcliff
I'm trying to understand the bold sentence. I think "which" refers to "the consciousness of the acting". This makes me understand the the whole idea as " only the consciousness of acting can impart this serenity to a man of moral like him"
I agree with your understanding of the idea, but given our agreement on that point, "which" must refer to "serenity over his moral manners." So: (1) nothing else (other than the consciousness of acting right) could impart serenity over moral manners to a man of moral perceptions like his, and (2) serenity over moral manners refined his sense of every surrounding blessing.
In simple English, I understand the text to mean:
The awareness of doing the right thing
left him with a calm, rewarding feeling
which nothing else could rival
in a man aware of right and wrong as he was,
and which helped him appreciate the good things of life.
