3 Nephi 20:
15 And I say unto you, that if the Gentiles do not repent after the blessing which they shall receive, after they have scattered my people—
16 Then shall ye, who are a remnant of the house of Jacob, go forth among them; and ye shall be in the midst of them who shall be many; and ye shall be among them as a lion among the beasts of the forest, and as a young lion among the flocks of sheep, who, if he goeth through both treadeth down and teareth in pieces, and none can deliver.
17 Thy hand shall be lifted up upon thine adversaries, and all thine enemies shall be cut off.
18 And I will gather my people together as a man gathereth his sheaves into the floor.
19 For I will make my people with whom the Father hath covenanted, yea, I will make thy horn iron, and I will make thy hoofs brass. And thou shalt beat in pieces many people; and I will consecrate their gain unto the Lord, and their substance unto the Lord of the whole earth. And behold, I am he who doeth it.
The "Gentiles" have had a great ride. One can only look at the advances and, frankly, blessings that have come to Western civilization over several centuries to know that God has smiled on this people. Sadly, Jesus here tells that the ride will come to an end.
As we have seen in the past few decades, most of the gentile peoples and nations have been turning away from Christ and his God, either recasting them as something other than the scriptural record says, or simply casting them aside altogether. In my mind, this is what they should "repent" of as referenced in verse 15. As this has not happened, the time is coming as spoken of in Luke 21:24, when the "times of the gentiles" will end.
Christ is very, very poignant in these Nephi verses that the children of Israel will be scattered among the gentiles and be very destructive to them. As a lion "treadeth down" and "teareth in pieces", relations between the followers of Christ and those who reject Jesus will be no peaceable "lion lying down with the lamb" situation!
Verse 19 makes it very clear: the Lord through the children of Israel will destroy "many people". Jesus wants to make it clear that he will be the one behind it all and everything gained in the destruction will be the Lord's. Then that last statement: "I am he who doeth it", which should remind us all, both those who conquer and those who are conquered, that this is done by Jesus' hand and by his means.
For those who do not repent of their lack of gratitude toward God, these results lie ahead: "none can deliver"; "all thine enemies shall be cut off". The time to repent of ingratitude and to join the family of Israel through authoritative baptism is NOW.
The time of the Lord's blessing of the Gentiles is nearly through and the time of the rise of righteous children of Jacob is about to begin. There is very little time left to join the Lord's side - the side of righteousness and gratitude.