Damis: "Good God! Do you expect me to submit To the tyranny of that carping hypocrite? Must we forgo all joys and satisfactions Because that bigot censures all our actions?" (1.1.18)
Damis comes right out and says what everyone is thinking. He drops the H-bomb mere minutes/pages into the play.
Dorine: "You see him as a saint. I'm far less awed; In fact, I see right through him. He's a fraud." (1.1.23)
This notion of "seeing right through" is an important one. Hypocrisy is a way of hiding one's own faults. In this case, Tartuffe doesn't seem to be doing such a great job of hiding his.
Dorine: "Those who have greatest cause for guilt and shame Are quickest to besmirch a neighbor's name. [...] By talking up their neighbour's indiscretions They seek to camouflage their own transgressions" (1.1.31)
Dorine lays out a simple motive for hypocrisy; it's a means by which one's own wrongs can ignored or swept under the rug.
Cléante: "There's a vast difference, so it seems to me, Between true piety and hypocrisy: How do you fail to see it, may I ask? Is not a face quite different from a mask?" (1.5.9)
Cléante is right that there is a big difference between true piety and hypocrisy, but he seems unable to acknowledge that sometimes even big differences can be hard to see.
Tartuffe (Observing Dorine, and calling to his manservant offstage): "Hang up my hair-shirt, put my scourge in place, And pray, Laurent, for Heaven's perpetual grace. I'm going to the prison now, to share My last few coins with the poor wretches there."
Dorine (Aside): "Dear God, what affectation! What a fake!" (2.2.1-2)
You don't need Dorine to tell you what a fraud Tartuffe is. He asks Laurent to put away his hair-shirt and scourge, instruments used to humble a person before God, in order that he might look holier in Dorine's eyes.
Tartuffe: "But soon, fair being, I became aware That my deep passion could be made to square With rectitude, and with my bounden duty." (3.3.27)
While Tartuffe is able to square his passion (lust for Elmire) with his rectitude, there's no doubt he would allow anyone else that kind of free pass. He, like any hypocrite, can always justify his own actions – even though, here, he does not let us know what his justification is.
Tartuffe: "Yes, brother, I'm a wicked man, I fear: A wretched sinner, all depraved and twisted, The greatest villain that has ever existed. […] Believe what you are told, and drive Tartuffe Like some base criminal from beneath your roof; Yes, drive me hence, and with a parting curse: I shan't protest, for I deserve far worse."
Orgon (To Damis): "Ah, you deceitful boy, how dare you try To stain his purity with so foul a lie?" (3.6.2-3)
Ironically, the one and only time Tartuffe actually tells the truth, he does so in order to trick Orgon once more. Unfortunately, his trick works.
Tartuffe: "How can you know what I might do, or be? Is it on my good actions that you base Your favor? Do you trust my pious face? Ah, no, don't be deceived by hollow shows; I'm far, alas, from being what men suppose;" (3.6.6)
Tartuffe's advice to Orgon is quite similar to Cléante's. If there's one thing you can trust Tartuffe to know, it's the ins and outs of hypocrisy. After all, he's got a lot of experience with them.
Tartuffe: "God knows what people would think! Why they'd describe My goodness to him as a sort of bribe; They'd say that out of guilt I made pretense of loving-kindness and benevolence." (4.1.2)
In order to justify his treatment of Damis, Tartuffe tells Orgon that he simply can't forgive him…because if he did, people might think he were a hypocrite.
Cléante: "Why weren't you moved to give your evidence Until your outraged host had driven you hence? I shan't say that the gift of all his treasure Ought to have damped your zeal in any measure; But if he is a traitor, as you declare, How could you condescend to be his heir?"
Tartuffe (To the Officer): "Sir, spare me all this clamor; it's growing shrill." (5.7)
Tartuffe has no answer for Cléante's allegations, and with good reason: Cléante has just pointed out as flagrant a case of hypocrisy as any. Tartuffe, who claims to be a moral man, is willing to be the heir of a supposedly immoral person. It's a real "gotcha" moment.