In order to fully appreciate this scene, you should recall that Olivia gave Sir Toby and the household staff orders to take care of Malvolio and the "midsummer madness" that turned him into a grinning zany, tightly cross-gartered, and garbed in yellow stockings. They locked him in a dark room, and now Maria and Feste prepare to pull a few more pranks on the supercilious, overbearing Malvolio. Feste disguises himself as a parson and plans to make a "mercy call" on the "poor mad prisoner." He will assume the role of Sir Topas, the curate. The interview is a masterpiece of low, broad comedy.
Feste, as Sir Topas, knows just enough Latin phrases to lace them into his interview, along with pedantic nonsense and pseudo-metaphysical drivel concerning the philosophy of existence. The imprisoned steward, of course, is extremely relieved to hear what he believes to be the parson's voice, for he fondly imagines that his deliverance from this darkened room of a prison is near. This is not the case, however; he will "remain in his darkness" for some time to come.
When Feste slips out for a moment, Sir Toby suggests that Feste use his natural voice to speak with Malvolio; things have taken a turn for the worse, and he wants to release Malvolio and end this charade. He is afraid that Olivia might turn him out of the house, and he "cannot pursue with any safety this sport to the upshot."
Feste is having too much fun, though, to pay much attention to Toby's fears; he enters Malvolio's room, assumes his ecclesiastical voice, and tries to convince the steward that there are two visitors in the room instead of one. Malvolio pleads that he is not insane, and finally Feste is persuaded to bring Malvolio some ink, a pen, and some writing paper so that he can "set down to [his] lady" proof of his sanity.
Once again, disguise is used to create comic effect. This time, Feste disguises himself as a parson and appears before Malvolio. The disguise utilizes a black gown, the same type of gown that Malvolio had worn earlier. The comedy is multifold: Malvolio thinks that with the appearance of the parson some light will be shed upon his insanity, but actually, Malvolio will have to remain in darkness for some time to come. As Feste says: "There is no darkness but ignorance," and certainly Malvolio was ignorant to think that Olivia could ever be attracted to him.