Now, repentance could not come unto men except there were a punishment, which also was eternal as the life of the soul should be, affixed opposite to the plan of happiness, which was as eternal also as the life of the soul.
Why is repentance not possible without eternal punishment? I understand why punishment must exist (it's explained well in the chapter), but it doesn't explain why the punishment must be eternal.
There are similar discourses elsewhere in the Book of Mormon, notably in 2 Nephi 2 and 2 Nephi 9. Especially in 2 Nephi 9:6-8 it is explained that without the atonement of Christ, the judgement that came on Adam would have been eternal and unending, but not why that is a precondition for repentance.
6 For as death hath passed upon all men, to fulfil the merciful plan of the great Creator, there must needs be a power of resurrection, and the resurrection must needs come unto man by reason of the fall; and the fall came by reason of transgression; and because man became fallen they were cut off from the presence of the Lord.
7 Wherefore, it must needs be an infinite atonement—save it should be an infinite atonement this corruption could not put on incorruption. Wherefore, the first judgment which came upon man must needs have remained to an endless duration. And if so, this flesh must have laid down to rot and to crumble to its mother earth, to rise no more.
8 O the wisdom of God, his mercy and grace! For behold, if the flesh should rise no more our spirits must become subject to that angel who fell from before the presence of the Eternal God, and became the devil, to rise no more.
It appears that Alma is using "repentance" as shorthand for the plan of redemption including repentance, as summarized in Alma 42:13. Otherwise, this particular verse is difficult to interpret.
D&C 19 has a really interesting explanation of the meaning of eternal that may also apply to its usage in Alma:
4 And surely every man must repent or suffer, for I, God, am endless.
5 Wherefore, I revoke not the judgments which I shall pass, but woes shall go forth, weeping, wailing and gnashing of teeth, yea, to those who are found on my left hand.
6 Nevertheless, it is not written that there shall be no end to this torment, but it is written endless torment.
7 Again, it is written eternal damnation; wherefore it is more express than other scriptures, that it might work upon the hearts of the children of men, altogether for my name’s glory.
8 Wherefore, I will explain unto you this mystery, for it is meet unto you to know even as mine apostles.
9 I speak unto you that are chosen in this thing, even as one, that you may enter into my rest.
10 For, behold, the mystery of godliness, how great is it! For, behold, I am endless, and the punishment which is given from my hand is endless punishment, for Endless is my name. Wherefore—
11 Eternal punishment is God’s punishment.
12 Endless punishment is God’s punishment.
In other words, as I understand these verses, eternal punishment does not mean a punishment that has no end, but punishment that is dealt out by God. Eternal is "more express" "that it might work upon the hearts of the children of men" for God's glory (v. 7).
