23 For behold, in that day that they shall rebel against me, I will curse them even with a sore curse, and they shall have no power over thy seed except they shall rebel against me also.It helps to know who all the antecedents are in this verse; there are a lot of pronouns.
“For behold, in that day that they [thy brethren] shall rebel against me, I will curse them [thy brethren and their seed, the Lamanites] even with a sore curse, and they [the Lamanites] shall have no power over thy seed [Nephites] except they [Nephites] shall rebel against me also.”
What does it take to rebel against the Lord? Consider the Lamanites, Laman and Lemuel in particular. As their story unfolds, we see them do some pretty bad things that would seem to indicate rebellion: beating up on Nephi several times, tying him up while crossing the ocean, trying to kill their father, the prophet. Verse 13 in this chapter says they were “like unto the Jews who were at Jerusalem, who sought to take away the life of [Lehi].” This all seems pretty rebellious. And yet, in verse 23 the Lord says he will curse them “in that day they that they shall rebel against [Him].” The Lamanites weren’t cursed, at least not outwardly with the dark skin, until it got so bad that Nephi had to leave with his people. (2 Nephi 5) I think at this point they were so far gone that there was no way they were repenting. So finally, the Lord cursed them. They got a lot of chances up until then, though. Clearly He keeps His own counsel to forgive often, as state in Matthew 18:21–22. The Lord expects more from those who covenant with Him.