Like all successful con artists, Tartuffe is a master in manipulating others; however, only Orgon and his mother, Madame Pernelle, fall for his pious act, unable to see Tartuffe's selfish motives. Why they are taken in has as much to do with who they are, perhaps, as it does with Tartuffe's talent for deception. That said, though, Tartuffe did exhibit much skill in deceiving, and ultimately betraying, Orgon.
Tartuffe's primary skill in manipulating Orgon is his ability to read Orgon's character and employ the strategy of reverse psychology. After first attracting Orgon's attention in church with his public displays of religious fervor and holiness, Tartuffe refuses to accept all of Orgon's charity; he will accept only a portion of what Orgon offers him, thus presenting himself to be the exact opposite of what he is: a person who seeks to enrich himself. Orgon is taken in by Tartuffe's clever behavior and brings him home.
Once he becomes a member of Orgon's household, Tartuffe maintains power over Orgon largely by encouraging Orgon to wield his power as the head of the house. By criticizing the behavior of Orgon's family and servants, Tartuffe plays to Orgon's ego. Orgon, after all, is the one has the power to control the others and must use it for their eternal salvation. Becoming a tyrant in his own family, then, is excused in Orgon's mind as a matter of duty.
When Damis witnesses Tartuffe's immoral behavior with Elmire and takes his "proof" to Orgon, Tartuffe again uses reverse psychology to manipulate Orgon. Tartuffe seems to defend Damis, and then Tartuffe says he is the worst of human beings, one who is not what he appears to be. By telling the absolute truth about himself, Tartuffe convinces Orgon that he is so holy that he would sacrifice himself for Damis. The result, of course, is that Tartuffe is revered even more by Orgon and Damis is thrown out of his father's house.
When Orgon finally does realize what a fraud Tartuffe is, he cannot believe he had been taken in so completely. Tartuffe had been very good at understanding Orgon's character flaws and manipulating them. Human nature had worked in Tartuffe's favor, as well. Once Orgon and Madame Pernell (his mother) had declared their allegiance to Tartuffe, it was very hard for them to admit they had been wrong and the other members of the household had been right all along.