Dorine: "To hear him talk – and he talks all the time – There's nothing one can do that's not a crime. He rails against everything, your dear Tartuffe." (1.1.19)
If Dorine has it right, Tartuffe's moral compass is way off. Even if he weren't sinning, that kind of attitude speaks to a misunderstanding of sin and religion in general. Also, she might have added "except the stuff he does" after everything if she wanted to be a bit more accurate.
Madame Pernelle: "Well, mark my words, your souls would fare far better If you obeyed his precepts to the letter." (1.1.23)
Once again, if we believe Dorine – and we must – following his precepts would really mean doing nothing at all.
Madame Pernelle: "You all regard him with distaste and fear Because he tells you what you're loath to hear, Condemns your sins, points out your moral flaws, And humbly strives to further Heaven's cause." (1.1.27)
Madame Pernelle's argument might be valid in another case – there are plenty of prophets that were ignored because people simply didn't want to change their ways. In this case, however, Orgon's family is right not to listen.
Dorine: "When there's a chance for libel, they never miss it; When something can be made to seem illicit They're off at once to spread the joyous news. Adding to fact what fantasies they choose." (1.1.31)
Dorine points out that sin, which deserves to looked down upon, can be seen as a cause for celebration, depending on how you look at it.
Dorine: "He [Tartuffe] ate his meal with relish, And zealously devoured in her presence A leg of mutton and a brace of pheasants." (1.4.10)
Don't forget: gluttony is one of the Seven Deadly Sins.
Orgon: "And how austere he [Tartuffe] is! Why, he can detect A mortal sin where you would least suspect; In smallest trifles, he's extremely strict." (1.5.6)
According to Orgon, Tartuffe is a kind of sin detector, uncovering hidden wrongs all over the place; he thinks this is really great, too. Did he forget about the whole "judge not lest ye be judged" thing? Sure seems like it. Oh, and I think he forgot about this little tidbit from the Gospel of Matthew too: "And why beholdest thou the mote that is in thy brother's eye, but considerest not the beam that is in thine own eye?" (Matthew 7:3)
Tartuffe: "Cover that bosom, girl. The flesh is weak, And unclean thoughts are difficult to control. Such sights as that can undermine the soul."
Dorine: "Your soul, it seems, has very poor defenses, And flesh makes quite an impact on your senses." (3.2.7-8)
Tartuffe's own piety and knowledge of sin springs from his inability to avoid impure thoughts.
Tartuffe: "I know such words sound strangely, coming from me, But I'm no angel, nor was meant to be, And if you blame my passion, you must needs Reproach as well the charms on which it feeds." (3.3.29)
As with Dorine, Tartuffe blames his own sinful thoughts on others, despite the fact that they are unaware of having any such effect.
Tartuffe: "I know, dear lady, that your exceeding charity Will lead your heart to pardon my temerity; That you'll excuse my violent affection As human weakness, human imperfection;" (3.3.31)
Tartuffe justifies his actions by claiming that they're simply human nature; mankind, he suggests, is inherently sinful or "imperfect," and so he can't be blamed for his error.
Orgon: "These papers vexed my conscience, and it seemed best To ask the counsel of my pious guest. The cunning scoundrel got me to agree To leave the strong-box in his custody, So that, in case of an investigation, I could employ a slight equivocation And swear I didn't have it, and thereby, At no expense to conscience tell a lie." (5.1.8)
Orgon's logic is hopelessly…illogical. After all, a "slight equivocation" is a still a lie. Furthermore, as long as he knows where the documents are, he still has them for all intents and purposes.