Blindness is a metaphor for not understanding spiritual things, and having sin is a metonym for being guilty of sin. Alternate translation: "If you truly were spiritually blind, you would not be guilty of your sin" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonym]])
Here "seeing" is a metaphor for understanding spiritual things, and "sin remaining" is a metonym for continuing to be guilty of one's sin. Alternate translation: "but since you think that you understand spiritual truth, you remain guilty of your sin" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonym]])