In act 2, Mary admits that she made the poppet in court to give to Elizabeth, and Abigail was fully aware of the poppet. A clear-headed individual would realize that Elizabeth did not bewitch Abigail, but the events in Salem were being driven by people not so clear of mind.
John's response is to order/convince Mary to testify to the court how the poppet came to be in his house:
You will tell the court how that poppet come here and who stuck the needle in.
I believe that this is the moment that you are meant to answer your question. Feel free to explain what you think. If you know the play, then you know the end result of what happens when Mary begins to tell what she knows. Mary's response to John is to tell him that Abigail will call lechery upon John. However, John knows that Abigail's admission of having sex with John will actually ruin her own reputation. It will help call into question Abigail's "evidence" and help to set Elizabeth free. Mary continues to protest, repeating, "I cannot, I cannot."
The judges and the rest of the court are not likely to receive Mary's testimony positively. By this point in the play, they have spent too much time and energy believing Abigail. Anything that they do or believe against Abigail ruins their own credibility and authority as judges of the court. Mary's testimony makes them look bad, so they are not likely to want to hear it.