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Isaiah 48 is the first chapter in the Book of Mormon copied (see 1 Nephi 20), and almost every verse is different in the Nephite record.
Isaiah 48:1
Swearing by the name of the Lord means making covenants.
1 Nephi 20:1
Hearken means to listen and to obey.
These are baptized members in Isaiah's day, but they are people who are not converted.
Isaiah 48:16
Why would he say that he has not spoken in secret? Isaiah 45:19 talks about it, too. I also like Isaiah 45:14. In an assembly of the people, the many voices, the Lord is among them. He does not use mysticism and speak in dark corners.