Rhetorical: Once again in his discourse Abinadi is about to get very personal. While we might read these verses in a modern context as a description of the resurrection, in the context of Abinadi's discourse this is a very direct accusation against the priests. Remember that his entire discourse has turned their accusations against him into a counter attack against the priests. From this point to the end of our chapter 16 is Abinadi's final prophetic condemnation of the priests. Abinadi began by declaring that they were worshipping the wrong god because they denied the atoning Messiah. His entire discourse has been to show how badly they have misinterpreted the texts, and now he shows that this denial of the atoning Messiah places them in condemnation.
Doctrinal: Abinadi now discusses the nature of the resurrection. First, he uses the resurrection of the Messiah as the grounding point for the resurrection of all of humanity. Isaiah's text allows only for the resurrection of the Messiah, so Abinadi is relying here on other revelations, most likely to Lehi, Nephi, and Jacob. He also presumes that the priests of Abinadi understand that the resurrection is for all, and not just the Messiah, as he continues with his argument without proving that particular point.
While resurrection is for all, it nevertheless has a component in it that divides the righteous from the unrighteous. While both share in the act of the resurrection, the nature of the resurrection (most likely implying the following judgement without so stating) differentiates between the righteous and the unrighteous. The first resurrection is for the righteous and the second resurrection for the unrighteous.