Complex sentence: The Carracci and their followers were condemned by Ruskin as being "insincere" (for Ruskin, there was no entirely sincere or great art in the seventeenth century), and doubly damned as being "eclectic".
Split into simpler sentences: The Carracci and their followers were condemned by Ruskin as being "insincere". For Ruskin, there was no entirely sincere nor any great art from the seventeenth century and all was doubly damned as being "eclectic".