What duty has Mary Warren been performing in Salem?

In Act Two, Mary Warren tells her employers, who are John and Elizabeth Proctor, that she is now "an official of the court."  She has been going into Salem in order to give testimony against accused witches such as Sarah Good, and she considers this to be her Christian duty.  When John Proctor forbids her to go into Salem to the courts again, she says,

I must tell you, sir, I will be gone every day now.  I am amazed you do not see what weighty work we do.

Despite Mary Warren's evident doubts––after all, she tried to convince Abigail Williams to confess in Act One––she has apparently, now, bought into the lies the girls are telling.  She tells stories about feeling "a misty coldness climbin' up [her] back" as she sat in court while Sarah Good denied the charges against her. Mary also believed that Sarah Good cursed her, after Mary had refused the beggar food and drink one day.  She believes that Goody Good witched her, because she had felt as though her "guts would burst for two days after."  Mary seems to believe in the things she's saying now, and she fully believes it is her Christian duty to go to the courts and provide such testimony during the trials of these people.