Verse 23 should be read with the end of 22: "And now behold, the Lord remembereth all them who have been broken off, wherefore he remembereth us also." The ability of the Nephites to "cheer up your hearts" is dependent upon the evidence that the Lord remembers them as well as the others on the isles of the sea. The promises of the Lord come to bring them cheer in their fulfillment.
Jacob now sets up the final call to covenant with a description of their position. They are free to choose, and the choice is before them; eternal life or eternal death. This dual "way" has precedent in the teachings of Jeremiah as well as Nephi:
Jer. 21:8 And unto this people thou shalt say, Thus saith the LORD; Behold, I set before you the way of life, and the way of death.
2 Ne. 2:27 Wherefore, men are free according to the flesh; and all things are given them which are expedient unto man. And they are free to choose liberty and eternal life, through the great Mediator of all men, or to choose captivity and death, according to the captivity and power of the devil; for he seeketh that all men might be miserable like unto himself.
The presence of the teaching of the two ways in Jeremiah, and Nephi's expansion on the theme, suggests that Nephi may have understood the doctrine from the brass plates, and presented it in the expanded form. Jacob could therefore have known it from either source, and it is probably that his audience also understood the reference.