These verses exist in prophetic(as opposed to historic) parallel to the vision that his father, Lehi, had where he described the mists of darkness and the multitudes pressing forward towards the tree of life (see 1 Nephi 8:24). It is beautiful, with this in mind, to learn that those that emerge from mists of darkness are brought into the presence of the Lamb of God.
... I saw the vapor of darkness, that it passed from off the face of the earth; and behold, I saw multitudes who had not fallen because of the great and terrible judgments of the Lord.
And I saw the heavens open, and the Lamb of God descending out of heaven; and he came down and showed himself unto them. (vs. 5 & 6)
Nephi then sees in vision the twelve disciples and three generations of his people following the appearance of the Savior. Both groups are distinguished as righteous and as having their garments made white in the blood of the Lamb of God. In both instances, Nephi, or the angel, points out that this cleansing of their garments is "because of their faith in him," (vs. 11). Furthermore, this is stated twice: first for the twelve ministers and then for the three generations.
These statements first establish the qualifications of the twelve disciples as ministers and judges of the people of Nephi. It then demonstrates that in the righteousness of a generation how a people can believe together and that salvation can be obtained by all.