The first phrase picks up on the last phrase of the previous verse, “whether they be unto salvation or unto destruction.” The duality of salvation/destruction is transmuted into the duality of good/evil. Taking the one opposite pair, he extends his meaning with a related and correspondingly opposite pair. Thus salvation or destruction come upon men, and all men have placed before them both good and evil. Since Alma has declared that God’s decrees are unalterable (verse 4), he now needs to show how the unalterable decrees apply to mankind that has the options of good and evil.
Alma notes that “he that knoweth not good from evil is blameless.” The salvation/destruction pairing is not applicable where men are not accountable for the good/evil pairing. Without being able to separate good from evil, they are not liable to be separated into saved or destroyed.
For the person who knows the difference between good and evil, to that person the full option of salvation/destruction applies. The person will receive the reward he selects, with good, life, and joy being equated, and evil, death, and remorse of conscience being the opposite conditions.
Alma’s teaching that “he that knoweth not good from evil is blameless” is a direct descendant of father Lehi’s instruction on the opposition in all things. As Lehi taught in 2 Nephi 2, if there is no opposition all things must be a compound in one (2 Nephi 2:11. See the commentary after that verse for a discussion of what a “compound in one” means). Very specifically, Lehi also notes:
2 Nephi 2:13
13 And if ye shall say there is no law, ye shall also say there is no sin. If ye shall say there is no sin, ye shall also say there is no righteousness. And if there be no righteousness there be no happiness. And if there be no righteousness nor happiness there be no punishment nor misery…
Flowing from Lehi’s teachings, a man who does not know good from evil must be blameless, for he would be unaware of the opposition. For a man who does not know good from evil, he must be as innocent as Adam and Eve before partaking of the fruit of the Tree of Knowledge of Good and Evil. Of course Alma is aware that all men must have some knowledge of good and evil as a result of the fall, but his argument is in the realm of the theoretical and verbally symbolic rather than the literal. He is creating the extreme position so that we will better understand the issue he is explaining. For Alma, once we know the decrees, we are firmly accountable to them.
Knowing the difference between good and evil does not guarantee salvation, however. Knowing good from evil we must actively choose good, so that good will be returned to us. This is the essential lesson that Paul was attempting to teach in Romans chapter 2:
Romans 2:12-13
12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
13 (For not the hearers of the law are just before God, but the doers of the law shall be justified.
In Paul, the “law” refers to the law of Moses by which the life of the Jews was governed. Paul is intent on making sure that the Jews understood that it was not the law itself that saved, but rather the “doers of the law.” Similarly, modern members of the church may take false comfort in the rituals of a social gospel. We might assume that our baptism into the church will make all of the difference for us. Just like the Jews, we may assume that the wonderful law that we have been given will, in and of itself, be our salvation. Paul reminds us that this is not so. We may have the law, we may (per Alma) understand good from evil, but it is what we do with that knowledge that will lead us to life and joy, or death and remorse.