We hear some strange opinions about sin in Tartuffe. Madame Pernelle tells everyone that Tartuffe is the authority as far as sin's concerned. Nobody listens of course…well, except for Orgon. Now, you may be wondering what separates "sin" from "morality and ethics." Truth is, they're intimately related. But in this case, it's a matter of specifics. Tartuffe manages to ignore so many real, Christian rules about proper behavior – even as he's pretending to be the authority on it – that you can't help but be reminded of the particular rules he's breaking.
However much of a wrongdoer he is, Tartuffe's actions actually teach us about sin, inasmuch as he shows us what not to do.
In the end, we learn that sins can and should be forgiven, that even Tartuffe has a chance at redemption.