This is the point of contention between Abinadi's religion and that taught by the priests. As noted, it is the Sherem religion, a brass plates religion devoid of the Messianic prophecies of the Nephite prophets. Also as noted, perhaps we should not be surprised to find this religion popular in the land of Nephi as it was in this very land that Sherem preached. If we remember that not all of those who were considered Nephites at the time left with Mosiah, and that those who remained were those most likely to have invited Sherem's preaching (see the discussion of Jacob 7) then we might see that brass-plate-minus-Messiah religion as the regionally dominant one, and one supported by those who probably spoke the same language as these newly returned peoples to this area. It may be no surprise that the change in religion could occur so quickly if it was the dominant religion of the area, and one that had roots similar to those of the people of Zeniff-now-Noah.
Abinadi's argument presupposes that the priests of Noah are aware of the Messianic teachings. He takes for granted that they have heard of the atonement by the Messiah. If Abinadi were one of Zeniff's priests as Tvedtnes has suggested, then he would know first hand that such things had been taught in this land. Abinadi proceeds not by revealing things that they have never known, but by explaining to them the things they have chosen to disregard. They sin not in ignorance, but by choice, and perhaps by avarice.
There are two important points that Abinadi makes. The first is that the atonement is the salvation, and the good that comes from living the law of Moses comes because of that atonement, not apart from it.
The second, and perhaps most interesting, is his statement that "God himself" will effect the atonement. It is clear from the following verses (particularly verse 34) that Abinadi is linking God and Messiah into the same being. This is not current LDS theological practice, but as indicated in the prior instances in the Book of Mormon, this conjunction of God and Messiah is a normal occurrence for this people surrounded by a polytheistic society. What is important is not the canard proposing a conflict with modern doctrine, but the internal consistency with Abinadi's slight alteration of "gods" to "God" in Mosiah 12:35. In that verse Abinadi shifts "gods" in Exodus 20:3 to the singular God. As was noted in the comment on that verse, it is possible that this is not intentional nor meaningful. However, it is also very possible that it is.
When we understand that Abinadi is speaking of the Messiah, and referring to the Messiah in terms of "God himself" atoning for man, we can see Abinadi setting up a situation in which the priests of Noah are teaching a false god because they are denying the Messianic role of that God. Thus Abinadi is very purposefully exposing the priests of Noah as having another god before the true God, the God-who-will-be-Messiah.
As a last point to clarify the theology, is Abinadi wrong? Is he missing something that we understand from modern revelation? Perhaps not. If we remember that the God of Abinadi was referred to as Jehovah, and that Jehovah is the premortal name of Christ, then it is absolutely true that God (Jehovah) himself came down among us and atoned for us.