While it is possible to discuss verse 5 without immediate reference to verse 6, to do so would violate Lehi's point. Lehi's discussion of law is specifically designed to lead to the necessity of the Messiah in the establishment of the "way" to which he has referred.
Lehi on the Light of Christ
While Lehi's discourse on the mission of the Savior is well argued. The introductory verse "and men are instructed sufficiently that they know good from evil" is sufficiently compact that one can only suspect that this sermon is highlighting teachings that Lehi has presented in the past. This issue of the instruction of mankind in the difference between good and evil, and the absolute assurance that such instruction is sufficient that they might know is a critical piece of information, and deserves much more attention that Lehi gives it here. The brevity of this introduction to such an important facet of the plan can only suggest that it is a reminder of a teaching that is preliminary to the main point of his discourse.
Lehi will build a complex argument for the need or a redemptive Savior based on his law of oppositions. A requisite for this entire theological system, however, is this principle that man is "sufficiently instructed that they know good from evil." It would be grossly unfair of God to place us in a position where we were required to choose between good and evil, and not be given the capability to distinguish. For example, suppose a completely color blind man were required upon penalty of death to choose between red and brown. If both "colors" appeared to him to be differing intensities of gray, how would he make a distinction?
It is critical, therefore, that we are guaranteed that we are capable of making this selection. Lehi states it simply. Moroni provides that same information in a larger context:
16 For behold, the Spirit of Christ is given to every man, that he may know good from evil; wherefore, I show unto you the way to judge; for every thing which inviteth to do good, and to persuade to believe in Christ, is sent forth by the power and gift of Christ; wherefore ye may know with a perfect knowledge it is of God."
Moroni's explanation is tied to an admonition that we are to judge - at least in the context of judging good and evil. Relevant to Lehi's assertion that were are taught sufficiently to know the difference, Moroni explains that the difference is so great as to be like the difference between night and day (Moroni 7:15). Thus we are to understand that it should be an obvious distinction to us.
Moroni further explains in verse 16 how that guarantee is made. The Spirit of Christ, also known as the Light of Christ, "is given to every man, that he may know good from evil...". Thus our capability to distinguish good and evil is directly related to the Light/Spirit of Christ, a gift given to all men. The Spirit of Christ is given to all men specifically because we must have an eternal measuring device whereby we are assured of being capable of understanding the difference between good and evil. Of course men on earth may decline to listen to promptings of the Light of Christ, but that does not mean that it is removed - only ignored.
Much later, John would underscore the same concept:
7 The same came for a witness, to bear witness of the Light, that all men through him might believe.
8 He was not that Light, but was sent to bear witness of that Light.
9 That was the true Light, which lighteth every man that cometh into the world.
In John we are given to understand that Christ is the light that "lighteth every man that cometh into the world" but we must correlate John's statement with that of Moroni to understand the function of this Light of Christ.
Lehi on the Effect of Law
Lehi's presentation of the effect of the law appears to be very pessimistic:
Contrary to typical Old Testament usage, Lehi's discussion of Law does not appear to be related to the Decalogue, but rather to a conception of eternal law, a portion of which is embodied in the Law of Moses. Lehi's concern is to explain the relationship of man to that law. Remember that he is building on the basis of our guaranteed ability to choose between good and evil.
In the development of Lehi's argument, we are first guaranteed the capacity to distinguish between good and evil, and then we are commanded to choose good. This is the law - the law of the gospel that intends to exalt us. Ironically, the immediate effect of the law is to condemn us, because we as mortals are incapable of living it in its entirety. Our failure before even a portion of the law condemns us. Thus "by the law no flesh is justified." The law cannot exalt us, because it is our violation of a portion of that law that places us in a position contrary to justification. Lehi explains clearly that this is his intent, for his next clause is introduced as a definition of the first: "by the law no flesh is justified; or, by the law men are cut off". The law serves to cut us off from God, not to bring us closer (again uniquely related to the fact that we will violate the law - he hasn't resolved the conflict yet).
Lehi's next sentence further defines our predicament:
Lehi's use of the pronoun "they" makes this a personal statement. While he does not clearly define who "they" are at this point, it is evident that he speaks of Adam and Eve (to whom he will return later in the discourse). Placed in the context of the Garden of Eden and the law, Lehi's division into two types of separation is understandable. The "temporal law" is the physical sense in which the Fall affected their daily lives. The "spiritual law" also cuts them off, but in a much more powerful sense. The temporal Fall brought difficulties in earning their daily bread, but the spiritual Fall brought a separation from all that was good (definitionally God and his presence). Therefore the result of the spiritual Fall was being miserable forever.
Modern man wants to continually exclaim "if..., but...." We want to explain that it needn't be so. Lehi also understood that important distinction, but to most dramatically set his theme, he must first describe the conditions of the world had their been no redemptive Savior.
Lehi's Redeemer
Verse 6 begins the turn of the stream of the discourse from the effects of a world full of agency to a world saved by Christ:
Notice that verse 6 begins with the word "wherefore", which specifically links the two verses in a complementary relationship. It is precisely because of the doom of a world without a savior that one is provided. "Wherefore, redemption cometh." Lehi explains that the need for a Savior was great - wherefore it is come, and is come in the person of the Holy Messiah, one prepared for that role, one "full of grace and truth."