Matthew Arnold somewhere says: "the pursuit of perfection, then, is the pursuit of sweetness and light". what is the role of "then? Does it denote "therefore" or anything else?
The meaning of 'then' in this context follows Merriam-Websters definition 3:
Without more from the original text its hard to tell if it would be more appropriately one of the sub definitions a-d but my best guess would probably be 3d.
as a necessary consequence