The idiomatic expression is used here to display a sense of familiarity. Its idiom is used to display the message of the speech - everything relies upon this question. In addition to this, the rhetorical questions highlight his aim to make the audience think of their actions. As well, the use of the collective pronoun, "our" could reflect the idea that Saunder's doesn't want to put all the blame on his youthful audience, but instead includes himself to portray that he has also make mistakes.