Alma moves from his general arguments to a specific one from the scriptures. The people of Ammonihah still profess to believe in the scriptures, meaning the plates of brass, but do not believe in the coming Messiah. Alma uses a story from those scriptures to show that the ordained and righteous priests understood the redemption. He turns to Melchizedek.
Genesis 14:18–20 give us all our Old Testament knowledge of Melchizedek, save that a Messianic hymn, Psalm 110, notes of the coming Messiah, or perhaps of simply a king in Israel: “thou art a priest for ever after the order of Melchizedek” (Psalm 110:4). That is all we know that would have been available to Lehi and his descendants.
However, Alma appears to have a little more of the story, which we will see in the next verses. At this point, he is setting up the story. The declaration is that the people of Ammonihah should be as the people in the days of Melchizedek. Rather than move immediately to the example, Alma takes a small detour and notes that Melchizedek was important because even Abraham paid tithes to him, and that because Melchizedek was one of the foreordained high priests, he understood the need for the Messiah’s redemptive act.