The Mets' improbable weekend sweep of the Nationals and their steamroller victory over the Marlins on Monday night have made them a first-place team in the National League East and prompted their constituency to believe all things are possible. Those with Mets memories that reach back to before last Thursday have a restored sense of: "It can be done if we just believe." Since that dreadful rain-soaked loss against the Padres, believe has become the primary word in the vocabulary of Mets partisans. Indeed, believehas displaced pitch as the operative verb in their thoughts.