The explanation continues with Jacob establishing a context for the participation of the Gentiles. They are participants in the direct conflict between the covenant people and those who fight against them. Jacob's last sentence is interesting as he provides a justification for the inclusion of the Gentiles. Assuming them to be believers (as he has qualified in verse 13) he specifically defines them along with a particular kind of Israel "the people of the Lord are they who wait for him..." Thus the Gentiles become part of the "people of the Lord" and therefore the blessings of redemption should come through them.
Historical analysis: While Jacob has introduced the theme of the Messiah, a theme on which he will later expound, yet he begins with a very different concept and introduction. Jacob's base text from Isaiah deals with the salvation of Israel by the Gentiles. In his introduction to this theme, Jacob explicitly tells his audience that the words of Isaiah have a direct relevance to them (and presumably in their time): "2 Ne. 6:5 And now, the words which I shall read are they which Isaiah spake concerning all the house of Israel; wherefore, they may be likened unto you, for ye are of the house of Israel."
From the standpoint of both the text, the theme, and the direct request by Nephi that the sermon be given, it is appropriate to consider the reason behind a discourse on the salvation of Israel by the Gentiles. Because of the post-Messiah dating of the future restoration, it is clear that Jacob both understands and is making clear that these events are long in the future. However, it is also very possible that at this stage in the existence of this new people, that there is a very relevant context that Jacob is allowing his audience to understand, without making direct reference to it (a mode in which Nephi also tended to use scripture - giving a particular story - such as the exodus from Egypt, and allowing the audience to spell out the very close parallels).
It is possible that Nephi has directed Jacob to give this particular sermon because there has been a joining of the original people of Nephi with "gentiles" in the land. If this is the actual social context of the sermon, the relevance of the sermon does become one of immediacy. Not only is it a conciliatory sermon that would smooth any possible ill-feelings toward a gentile population in their midst, but it puts those very people in the context of being the salvation of the lineal Israelites, and allows them the blessings of Israel through the prophetic linkage to those future gentiles who are awaiting Christ.
This potential social background gives a very interesting context to Jacob's words in verse 13: "2 Ne. 6:13 Wherefore, they that fight against Zion and the covenant people of the Lord shall lick up the dust of their feet; and the people of the Lord shall not be ashamed." This sermon comes after the time that Nephi has already told us of wars and contentions with the Lamanites, and not we have Jacob giving a very explicit sermon about the gentiles being part of the fight, and on the side of Israel, and the means of redemption. I suggest that this is the most logical reason for the sermon, and the context in which the particular passages from Isaiah make the most sense.