The Book of Mormon proverb

20 And inasmuch as ye shall keep my commandments, ye shall prosper, and shall be led to a land of promise; yea, even a land which I have prepared for you; yea, a land which is choice above all other lands.
21 And inasmuch as thy brethren shall rebel against thee, they shall be cut off from the presence of the Lord.

Sounds familiar? If it does, it is because it is consistently repeated throughout the pre-Christ phase of the Book of Mormon. This is from 1 Nephi 2, the first time this statement is spoken by the Lord himself to Nephi while he was in the wilderness. This is also what Nephi refers to in 1 Nephi 4 when he is back in Jerusalem to get the brass plates

14 And now, when I, Nephi, had heard these words, I remembered the words of the Lord which he spake unto me in the wilderness, saying that: Inasmuch as thy seed shall keep my commandments, they shall prosper in the land of promise.

It is repeated several more times in 1st and 2nd Nephi
1 Nephi 17:13

wherefore, inasmuch as ye shall keep my commandments ye shall be led towards the promised land

2 Nephi 1:9

Wherefore, I, Lehi, have obtained a promise, that inasmuch as those whom the Lord God shall bring out of the land of Jerusalem shall keep his commandments, they shall prosper upon the face of this land

(It seems that both Nephi and Lehi had received this promise individually directly from the Lord. The "have obtained" refers to a previous occasion probably recorded on the large plates. A couple of times in Alma, this "Book of Mormon proverb" is referred to as something the Lord told Lehi.)

2 Nephi 1:20

And he hath said that: Inasmuch as ye shall keep my commandments ye shall prosper in the land; but inasmuch as ye will not keep my commandments ye shall be cut off from my presence. 

2 Nephi 4:4

For the Lord God hath said that: Inasmuch as ye shall keep my commandments ye shall prosper in the land; and inasmuch as ye will not keep my commandments ye shall be cut off from my presence.

2 Nephi 5:20

Wherefore, the word of the Lord was fulfilled which he spake unto me, saying that: Inasmuch as they will not hearken unto thy words they shall be cut off from the presence of the Lord. And behold, they were cut off from his presence.

Interestingly, after the books of Nephi, each of the small books contain this same statement. They write so little, yet they make sure to repeat this particular promise.

First, a variant of this spoken directly by the Lord to Enos in verse 10

I will visit thy brethren according to their diligence in keeping my commandments. I have given unto them this land, and it is a holy land; and I curse it not save it be for the cause of iniquity

Then a reference to what the Lord had said to "our fathers" in Jarom verse 9. We note the plural and the fact that Jarom's father, Enos, his granduncle, Nephi, and his great-grandfather, Lehi, had all received this promise individually

But the word of the Lord was verified, which he spake unto our fathers, saying that: Inasmuch as ye will keep my commandments ye shall prosper in the land

Another reference to what the Lord had said to "our fathers" in Omni verse 6

he would not suffer that the words should not be verified, which he spake unto our fathers, saying that: Inasmuch as ye will not keep my commandments ye shall not prosper in the land.

Over to the large plates of Nephi. I will not quote every single one but here is an overview:

• King Benjamin to his sons in Mosiah 1:7
• King Benjamin to the people in Mosiah 2:22 and 31
• Alma to the people of Ammonihah in Alma 9:13
• Alma to his son, Helaman, in Alma 36:1 and 30 (beginning and end of the chiasmus)
• Alma to his son, Helaman again in Alma 37:13
• Alma to his son, Shiblon, in Alma 38:1
• References by Mormon in his abridgment in Alma 48:15, 25 and 50:20

Altogether 20 repetitions of this promise. That is a lot! One may wonder why this is repeated so consistently from 1 Nephi through Alma and then it seems to disappear except a few possible allusions, like Helaman 12:21

And if the Lord shall say—Because of thine iniquities thou shalt be cut off from my presence—he will cause that it shall be so.

Why this is the case, I don't know. But the rest of the Book of Mormon is a fulfillment of this promise. After the Saviour's visit in 3rd Nephi, the people keep the commandments and prosper as promised in 4th Nephi

7 And the Lord did prosper them exceedingly in the land

Then they turn away from God, and are cut off from his presence (see for instance Mormon 1:14)

And there were no gifts from the Lord, and the Holy Ghost did not come upon any, because of their wickedness and unbelief.

Finally, the Nephites, as we know, are completely cut off from the land.