Verses 46 and 47 are unique to the Book of Mormon, and do not include the reference to publicans (see Matthew 5:46-47).
These verses conclude this set of lessons by linking them to the nature of the law. Nephi had preached that the Messiah would fulfill the law: “And, inasmuch as it shall be expedient, ye must keep the performances and ordinances of God until the law shall be fulfilled which was given unto Moses” (2 Nephi 25:30). Jehovah’s voice from heaven at the end of the three days of darkness declared the long-awaited event had come to pass: “In me is the law of Moses fulfilled” (3 Nephi 9:17).
Therefore, at the end of this section of the sermon that has been explaining how the law of Christ would related to the law of Moses, Jesus reiterates that the “those things which were of old time, which were under the law, in me are all fulfilled.”
Verse 47 echoes the words of Paul: “Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new” (2 Corinthians 5:17).