Alma hammers home his point. If they believed scripture, then they must believe in the Messiah. This argument tells us more about the regional religion before the Zoramites arrived.
Alma argues that, if they had read the scriptures, then they would have believed them—speaking specifically of Zenos and Zenock, two prophets recorded on the brass plates. The phrase “Son (of God)” was ambiguous in the Old World context but not for Alma, since he knew Nephi’s teachings, which were very clear about the Savior’s birth. The other Nephite prophets had been equally clear. But Alma is relying on the brass plates, not the Nephite prophets. As we have seen, many Nephite apostates, consonant with rejecting the Atoning Messiah, tended to accept the brass plates but not the later Nephite prophets—for example, Noah’s court and the order of the Nehors. (See Mosiah 12:25–29 and accompanying commentary for Noah’s court; see Alma 13:20 for Ammonihahite possession of scriptures.)
Furthermore, Alma’s emphasis on the Atoning Messiah answers this people’s question about whether they must believe in “one God” (v. 1). This “one God” is the Atoning Messiah and Alma’s listeners are familiar with the brass plates but not (or do not accept) Nephite prophets. We do not know if this group had an apostate version of the Nephite religion or whether their understanding of the brass plates came from a different source. (Perhaps some remembrance of the brass plates was retained among some Lamanites). In any case, their religious foundations are not recent, as Alma is not speaking of the Atoning Messiah as a concept they have rejected but rather one that presumably was never in their religion. This deduction provides further evidence that they had been in the locale before the Zoramites’ arrival.