The Sermon on the Mount contains an explicit statement of the relationship between Jesus and the Law of Moses. That statement was important in the Old World because it was the subject of discussion as to how to live the Law of Moses, and discussions over the purity Laws was particularly important. In the New World the prediction had been that the Atoning Messiah would come and a change would happen with the Law. Therefore this question would have been much more pressing in the New World context. The people would certainly have a question about the nature of the change that Jesus was introducing, therefore it is quite appropriate that as Jesus turns from laying the foundation of his gospel as taught in both hemispheres, he would concentrate on the issue that was most on the minds of the Nephites.
As Jesus begins this section of his discourse, he introduces the theme by suggesting that there is a saying with which the Nephites are familiar that they do not completely understand. We do not know how they knew this statement, as it is not contained in any of the Book of Mormon as it has come to us. The words used to couch the saying are most directly related to Paul:
2 Corinthians 5:17
17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
The contexts of the Pauline statement and the Book of Mormon usage of that same vocabulary is very different. In Paul it is suggestive of individual transformation through the gospel. In this setting in Jesus’ discourse, it becomes applicable to the transition from the Law of Moses to the Law of the Gospel.