Why does John insist that Mary Warren testify in Elizabeth's defense?

John's wracked with guilt over Elizabeth's arrest. He feels that he's in some way responsible—which, to a large extent, he is. The level of hysteria has reached fever pitch in Salem, destroying lives and setting neighbor against neighbor. John knows just how serious this is; he knows that Elizabeth is in real danger of being arraigned as a witch. He's got to do whatever it takes to save his beloved wife from such a terrible fate.
Somehow, John has to try and undermine the credibility of Abigail Williams's testimony. That shouldn't be too hard, one might think, given that her whole story has been nothing but a tissue of lies from the start. But Abigail's become a very powerful figure in Salem thanks to the witch-craze she started, and she's not about to relinquish the power she's accrued to destroy others. Truth be told, people are scared stiff of her, and with good reason, too. Just one word from her lying mouth and some poor soul could end up dangling on the end of a rope.
Mary Warren's only one of many townsfolk absolutely petrified by Abigail and the diabolical power Abigail wields with such wanton recklessness. She knows full well that Abigail's made up the whole witchcraft business, but she daren't let on. When John pleads with her to testify against Abigail, Mary flatly refuses, saying that Abigail will turn against her immediately and falsely accuse her of being a witch instead.