Lehi continues to develop his reasoning on the nature of the law and the atonement. After setting up the need for the atonement as a means of overcoming the penalties of the law, he here discusses the opposite end of the spectrum. Where he first describes the problem of the law superseding mercy, he now briefly discusses the problem of mercy supplanting law.
The beginning sentence is a truism. If there is no law there is no sin. This seems quite logical, as sin might have one definition that makes it a violation of eternal law. Lehi purposefully begins his exposition this way because there might be some who would think that a good idea. They might think that the absence of sin would be a good thing. Certainly that concept was implicit in Satan's thinking where he would prevent the pains of the violation of law, and therefore have no sin (implied in Moses 4:1).
Lehi emphatically denies that the absence of sin is of any benefit. In Lehi's equation, the absence of sin means also the absence of righteousness. To clarify the problem with the supposition that the absence of sin is beneficial Lehi continues the logic is parallel phrasings that each build on the previous idea:
Note that the phrases in parentheses are implied rather than explicit. This parallel set of examples is highlighting Lehi's proposition that there must be an opposition in all things, and that this opposition is so significant as to be the purpose behind all creation. The reason for its importance is specified in the last clause of verse 13. Lehi states "there could have been no creation of things, neither to act nor to be acted upon; wherefore, all things must have vanished away." the existence of opposites creates a condition where there are "...things, ... to act [or] be acted upon..."
Lehi's essential teaching is not simply the existence of the opposites, but the function of the opposites in creating situations where "things" would either act or be acted upon. While we typically concentrate on the oppositions, it is really this "acting" that is the powerful argument Lehi expounds. The opposites merely set the field for the "acting upon."