Moroni asks of the non-believers a question. Will they believe when Christ comes again in the last days? In the face of the actual presence of the Lamb of God, will they be able to deny the Savior? This may seem to be a very simple and simplistic argument. Of course they would not deny the Savior should he come in his glory to them. That is not really the issue that Moroni is asking. The implicit argument here is that there is evidence that the Messiah has come.
Moroni understands that the coming of the Messiah to the New World is not the same as the second coming, but he also understands history as being patterned. What has gone before is predictive of what will come. Therefore, Moroni’s argument really is that they would not fail to believe when he comes in the future, but they need not wait for that event to know. Since he as come before, he will come again. It is this use of the past to predict the future that will be the underlying theme as he develops his discourse to the future non-believer in Christ.