Surefire stories don't write themselves. That's the take-away from Amazing Grace, the epically bad show that opened Thursday at the Nederlander Theatre...If you're going to make a musical about one of the best-known and most-loved songs of all time, you'd better be equal to the task. That, fundamentally, is not the case with the this earnest but cringe-inducing freshman effort by composer-lyricist Christopher Smith...All the principal actors, and particularly Mackey and Cooper, have gorgeous voices...The closing scene is as powerful as it is predictable, as the hymn builds to its climax, sung by the entire company (and many in the audience). Everything that has come before is instantly forgotten.