Rhetorical: Jacob's introduction is not only brief, it is startling. Before he gives any of the specifics, he is very clear that this is a sermon to condemn. Jacob states that he gives the sermon under his responsibility to God, thus declaring that these are not Jacob's words, but the admonition and condemnation of their God. Secondly, he very specifically notes that he must "rid his garments" of their sins. This places Jacob and his audience on two different planes immediately. They are under condemnation, Jacob is not, and Jacob is fulfilling a responsibility to God that will ensure that Jacob remains apart from their sinfulness.
Social: We are justified in reading some social information into Jacob's declaration that this sermon is given on temple grounds. Jacob notes that he has "come up into the temple." Just as the land of Jerusalem included an area around the outside, the temple proper would consist of both the physical building and an attached area near the front of the temple. Since Jacob is speaking to an assembled body of his people it is most logical that the audience is congregated in this temple courtyard. Jacob himself would be at some vantage point closer to the temple, probably on steps of the temple. If the temple at this point in time had already shifted to the Mesoamerican model rather than the model of the temple of Solomon (and there is no indication that it did) it would be easy to see Jacob standing on the steps leading to the top of the pyramid. Even with the Solomonic model, there were probably steps which would elevate Jacob so that he could be seen by the audience.
In any smaller village, we would expect that there would only be two buildings appropriate for a formal rhetorical event, the political seat and the religious seat. We need not wander far from the Solomonic temple model to understand that the temple would be considered a religious location, and there would be a separate building for political events. Thus this particular sermon occurs at the temple emphasizing the religious, not the political nature of the discourse. Indeed, there may not have been any formal legal regulations governing the topics Jacob will address. In particular, it is doubtful that Jacob's admonitions against the pride of wealth would have had any legal sanction. Thus Jacob is, by his position, and by his selected location, providing a moral and religious condemnation that may not have been represented in the legal code of the village at that time.