The topic sentence, "Martin Luther King Jr.'s speech 'I have a Dream' has many brilliant examples of metaphorical language," is a subjective sentence.
In order to make it an objective sentence, you would need to remove the word brilliant (a). If you are objective about something it means you don't have any personal feelings towards it. Therefore, you are not stating that there are simply examples of metaphorical language, but don't have an opinion whether they are brilliant or not.