Nephi says that the Nephites have “have forgotten [their] God in the very day that he has delivered [them].” Typically, Nephite prophets have considered “deliverance” to be a deliverance from some military conflict, or perhaps a position of bondage. The most recent deliverance was the return of Nephite lands after the Lamanite conversion. They had been lost, and were returned. While the deliverance did not come through military action as it had in the past, it did come through the hand of the Lord, and very. Nephi was intimately involved in that conversion, and therefore understood the connection between that conversion and the deliverance of the Nephite holdings in the land of Zarahemla.
The specific catalogue of sins listed in verse 21 indicates that Nephi is directly preaching against the secret combinations. This suggests that the assembled crowd would understand what he was talking about. Certainly there were members of the Gadiantons present in the crowd (see Helaman 8:1), but it is also quite probable that the concept of the secret combinations was well known, even if the particular oaths were secret. The Gadiantons were, after all, a political party at this time, and in charge of the government. It would be surprising if the rest of the people did not know at least generalities about them. Part of the reason that the Book of Mormon’s secret combinations were presumed to refer to Masons by many readers is precisely that the Masons were known, and known to have secret oaths, even when the specifics were not known. In this same way, the Gadiantons at this point in time were hardly a secret organization, even though they did have secrets in the organization.