Each of these verses requires extensive exposition, but it is important to at least begin by reading them jointly, for they form a complete thought. Verse 12 is the conclusion of the lesson begun in verse 11.
Verse 11 is on of the well known verses in the Book of Mormon, but most well known in the version shortened to the first sentence. We are well aware that Lehi taught that there is an opposition in all things, but the rest of the verse is much denser language, and harder to understand. To begin the process of understanding the verse, let's examine the way it is composed.
There are four sections to verse 11, two statements, and two proof sets. The first statement is that there is an opposition in all things. Following that statement is the first proof set, a series of phrases intended to demonstrate the reality of the first statement. The second statement is that without opposition all things would be compound in one, followed by the set of proof phrases intended to demonstrate the validity of that statement.
Implicit in the verse is another structural "opposition." Very clearly the first statement and the second statement are also to be seen as opposites, or in this case mutually exclusive propositions. If we have the first we do not have the second, and if we don't have the first, then we do have the second.
Because the concept of opposition might be the more challenging (for all that it appears to be so clear), let's begin with the second. What does Lehi mean that everything "must needs be a compound in oNephi" Lehi's explanatory text, "wherefore, if it should be one body it must needs remain as dead, having no life neither death, nor corruption nor incorruption, happiness nor misery, neither sense nor insensibility," does not appear to be altogether that helpful. Nevertheless, we can discern somewhat his intent.
In each of the proof statements, Lehi indicates that divisions that we would take for granted would not exist. There is no life nor death, there is no corruption nor incorruption, no happiness nor misery, no sense nor insensibility. In each case, Lehi holds up examples that we would understand to be clear opposites, and says that under the conditions of a "compound in one" they would no longer be opposites, but rather the same, or at least that there would be no difference between them.
His exposition of concepts allows us to clarify the use of the phrase "wherefore, if it should be one body...." Lehi is not speaking of a body linked to a being, even though that phrase is linked to the life/death opposition. Lehi's reference is still the "compound in one" and it is that compound that is one body, or entity, or composite element. The import of Lehi's second statement is that there is a condition mutually exclusive to the opposition in all things in which there is no opposition, and where everything becomes as one thing, with no important distinctions between them. While there might be a life or death, there is no difference that matters. While there might be emotions that otherwise would register happiness or misery, there is nothing that gives them meaning.
Perhaps the best example of this is a soundless excerpt of a movie where we see a woman crying. With no context, we see tears. However, we know from our experience that tears can be for anger, sadness, or joy (and sometimes simply an irritant in the eye). Perhaps we might see other minute facial signs that would lend the context, but the fact is that the tears themselves can easily exist and have an ambiguous context. Thus Lehi would say that they would be a "compound in oNephi" There would be tears, but there would be no meaning to them. They would be water.
Understanding Lehi's "compound in one" we can now turn to the statement to which it serves as a contrast. What does Lehi mean by opposition? Lehi's proof statement is more of a statement of effect than a definition: "If not so, my first-born in the wilderness, righteousness could not be brought to pass, neither wickedness, neither holiness nor misery, neither good nor bad." Rather than define opposition, Lehi merely states that it is required, and that it is so essential that without it "righteousness could not be brought to pass." When elucidating that which could not come to pass, Lehi does provide examples that we consider opposites, such as holiness and misery, good and bad. However, it is important to remember that these are exemplary only of the things that could not come to pass. Lehi's point is that righteousness could not come to pass. I suggest that the real important phrase in his exposition is "come to pass," not any of the oppositions.
Lehi's choice of terminology is a literary device. He is categorizing a set of conditions by the most obvious members of the set. Lehi indicates that we need opposites, because they are the most obvious ways to define the range. Lehi does not mean that we require only happiness or misery. We also require every emotion in between. Lehi does not mean that we must be either good or bad, but that we must also have good and better. Lehi's point is the availability of choice, not the specifics of the choice. Opposition is his device to make that point in a concise discourse, where the obvious example serves stands for all of the less obvious examples.
Thus we have verse 12 (repeated here because the first is now so far removed from this text):
Verse 12 follows on the heels of Lehi's discourse on the "compound in oNephi" It is that compound in one that would defeat the purpose of the creation of this world. It is a violation of the will of God so severe that it "must needs destroy the wisdom of God and his eternal purposes." Thus by showing the powerful calamity of the "compound in one" Lehi underscores the absolute essentiality of the "opposition in all things."
Brigham Young explains the way in which we should see Lehi's concept of opposition: "You will learn this in the Bible, the Book of Mormon, and in the revelations given through Joseph. We must know and understand the opposition that is in all things, in order to discern, choose, and receive that which we do know will exalt us to the presence of God. You cannot know the one without knowing the other. This is a true principle. [DISCOURSES OF BRIGHAM YOUNG Page 346 ]
The principle of opposition is the principle of choice. We must make choices, but in order to make them, the possible choices must be made. It is this active participation in the selection of alternatives that exalts us. It is the reason that Lehi suggests that "righteousness could not be brought to pass." The principle of opposition does not in itself exalt us. It merely provides the options among which we exercise our agency, and it is the exercise of that agency that makes the difference.
Bruce R. McConkie understands this verse precisely in the view of agency: " Agency underlies all things--all advancement, all progression, even existence itself. It is based on the presence of opposites between which a choice must be made. If there were no opposites, there would be nothing. [A NEW WITNESS FOR THE ARTICLES OF FAITH , Page 90 ]"