When Othello confronts Emilia in Act IV about Desdemona's unfaithfulness, he has already decided that his wife has been unfaithful. Therefore, he seeks information, or evidence from Emilia about when and how often his wife has been unfaithful. He does not want to hear Emilia vouch for Desdemona's virtue and honesty with regard to her devotion to fidelity.
She tells Othello: "if she [Desdemona] be not honest, chaste, and true, / There's no man happy; the purest of their wives / Is foul as slander" (if Desdemona is not honest, chaste and true, then no man is happy as even the purest of their wives must be as foul)", (Line 16).
Othello responds by calling Emilia a simple woman, someone who does not understand the complexity of his inquiry. He then adds that Emilia is "a subtle whore, / A closet lock and key of villainous secrets;" yet a women who would kneel and pray since Othello explains that he has seen her do this (Lines 20-22).