When Elizabeth is arrested, John Proctor is no longer willing to stand by and wait for the courts to make the right decision. He pulls Mary Warren in and tells her
You're coming to the court with me, Mary. You will tell it in the court.
Here, Proctor is commanding Mary Warren to come to the court and tell what she knows. Mary Warren made the poppet that is being used as evidence against Elizabeth; she knows that Abigail saw her make the poppet and store the needle for safe keeping inside it. Because of this she knows Abigail is lying about Elizabeth. It was not her spirit,but Abagail's attempt to get to John.
Mary Warren is terrified to have to go to court and speak out against Abigail. She at first refuses telling Proctor, "I cannot- they'll turn on me-" because she knows the power Abigail has over the girls. However, Proctor convinces her that it is the right thing to do and she concedes.