This is an oft-quoted verse, but one that is difficult to understand in spite of its familiarity. What is “a compound in one?” It is a critical understanding for why the plan for life on Earth required the plan for a fall and a redemption before ever the Earth was begun.
First, there must be an opposition in all things. Why? This returns to Lehi’s earlier statement in verse 5, that all men were instructed sufficiently to know good from evil. What would happen if humankind not only did not understand the difference, but that there was no difference? It is not simply part of the plan that we should know good from evil, but that we should actively choose good over evil.
As an example, let’s say we are presented with two manufactured objects which are identical, and we are asked, upon penalty of death, to choose the better of the two? How could we do it? It is impossible to make an intelligent choice if there is a discernible difference. Lehi declares that the plan assures both that there is a difference, so there is the possibility of choice, and also that we have the divinely declared ability to choose the good.
Why does this matter? Just as Lehi noted, the condition of the heart is important when approaching Christ’s atonement. Our heart is changed to align with God’s principles to the extent that we make active choices of the good. We become good, not by getting a perfect score on a multiple-choice test, but by changing our hearts so that we are good.
“A compound in one,” therefore, is an unchoosable choice, the antithesis of opposition in all things. It is a condition where our agency is invalidated because there is nothing to work with. If nothing is better or worse, we cannot be changed by our choices, because there is no choice to make. “A compound in one” has made the choices for us by eliminating the necessity to make a choice. Therefore we remain as dead, because the purpose of our life has been removed.