Why is John Proctor arrested?

In act 3, John Proctor arrives at Salem's court with Mary Warren and a petition signed by landholding citizens who support Elizabeth, Rebecca, and Martha. John Proctor attempts to prove the witch trials are corrupt by having Mary Warren tell the truth and expose the girls as frauds. Unfortunately, Proctor is met with resistance and the court officials accuse him of attempting to undermine the court's authority. They end up dismissing Proctor's petition and also arrest Giles Corey when he presents a deposition against Thomas Putnam. In a final attempt to ruin Abigail's reputation and prove that she is a fraud, Proctor publicly admits that he had an affair with her. Unfortunately, Elizabeth lies to the court on John's behalf and Abigail begins to act like Mary's spirit is attacking her. Mary becomes overwhelmed with emotions and accuses John Proctor of colluding with the devil. When Deputy Governor Danforth begins questioning Proctor about his affiliation with the devil, John cannot control his anger and yells, "God is dead!" John is then arrested after making his declaration and is charged with colluding with the devil and attempting to undermine Salem's court.

In act 3, John Proctor comes to the court with a deposition containing nearly a hundred signatures of people who are willing to serve as character witnesses for Elizabeth Proctor, Martha Corey, and Rebecca Nurse. This is an act that does not endear him to the authorities. When Proctor continues to push for his wife's release, he is forced into the position of confessing his adulterous affair with Abigail Williams. This is enough to get him arrested (and executed under Salem's law), but it is not until Mary Warren accuses him of witchcraft that Proctor is actually arrested. Mary Warren points at John Proctor in the courtroom and says, "You are the devil's man." Because Proctor is frustrated at the illogicality of the proceedings (particularly the admission of spectral evidence) and the fact that the girls' accusations are condemning innocent people, Proctor yells, "God is dead!" and the marshals take him into custody.