Moliere's Tartuffe hinges completely on the aspect of sight or, more specifically, perception. Orgon is deceived because he cannot (or refuses) to see Tartuffe for the con artist he is. Tartuffe is hugely successful (for most of the play) because he is so good at wearing a mask of piety to cover his deception.
[Orgon] regards Tartuffe as his religious guide and is blind to the fact that Tartuffe is deceiving him.
Imagine the Paris home of Orgon, who meets Tartuffe at church and is completely taken in by him...so much so, that he foolishly not only invites this relative stranger, Tartuffe, to live in his home, but also promises his daughter (Mariane) in marriage to the man, though she has promised her heart to Valère.
The play centers on what is seen and what is believed. Orgon believes what Tartuffe wants him to believe—Tartuffe paints himself as a holy and virtuous man. Orgon cannot judge Tartuffe beyond the image he has of him in his mind even though he is cautioned repeatedly by several characters. His brother-in-law, Cléante, warns Orgon. Dorine, lady's maid to Mariane—outspoken not only as a woman, but also as a member of the hired staff—berates Orgon for not seeing Tartuffe for what he really is. Even Orgon's own son, Damis...
...is the most outraged by Tartuffe's behavior, reacting impulsively and threatening violence on several occasions.
Even when Damis hears Tartuffe attempting to seduce his stepmother (Elmire) and tells his father, Orgon will not listen, throws his son out of the house and threatens to disinherit him altogether.
The staging of the play is particularly important in the scene where Orgon's wife tries to convince him that Tartuffe is a fake—not the devout man he pretends to be. Elmire pretends that she loves Tartuffe, having hidden Orgon beneath the table in the room. Because Orgon cannot envision Tartuffe to be the man everyone claims he is, his wife must demonstrate the truth of Tartuffe's character by allowing Orgon to hear from the charlatan's own mouth what kind of fraud he truly is. Because Orgon is unable to see beyond Tartuffe's feigned saintliness, it is only after hearing the man not only make advances toward his wife, but also dismiss his religious beliefs that Orgon is able to step back, distance himself from the pretense Tartuffe has surrounded himself with, and call Tartuffe out as a scoundrel. Elmire lays the trap for Tartuffe about his faith—how can he contemplate seducing her, when he says he is devoted to Heaven?
But how can I consent to what you wish,
Without offending Heaven you talk so much of?
If Heaven is all that stands now in my way,
I'll easily remove that little hindrance;
Your heart need not hold back for such a trifle.
In perhaps the most entertaining scene of the play, Elmire convinces her husband to hide under the table to listen to this discussion. Only after witnessing Tartuffe's licentious behavior can Orgon understand for himself what almost everyone around him has been saying since he brought Tartuffe home.
Nothing more wicked e'er came out of Hell.