In a recent post, I mentioned that the plan of mercy is fundamental to the creation. It's one reason why the creation falls apart around those who reject the plan of mercy.
In this post, I'd like to give a few more examples of how the concept of mercy requires mortality. Mercy can only have claim over justice in the context of agency, repentance, death, and atonement.
The Fall brought about agency
A true test of our agency could only happen away from God's presence. The fall brought this about. Death allows for the probationary period to be temporary. Without the fall and death, God's word would be void (one of the descriptors of the pre-creation state of the earth from Genesis 1:2).
Alma-2 teaches this twice in Alma 12:
23 And now behold, I say unto you that if it had been possible for Adam to have partaken of the fruit of the tree of life at that time, there would have been no death, and the word would have been void, making God a liar, for he said: If thou eat thou shalt surely die.
...
26 And now behold, if it were possible that our first parents could have gone forth and partaken of the tree of life they would have been forever miserable, having no preparatory state; and thus the plan of redemption would have been frustrated, and the word of God would have been void, taking none effect. (Alma 12)
And once more in Alma 42:
5 For behold, if Adam had put forth his hand immediately, and partaken of the tree of life, he would have lived forever, according to the word of God, having no space for repentance; yea, and also the word of God would have been void, and the great plan of salvation would have been frustrated. (Alma 42)
The creation is the stage upon which the fall (and subsequent redemption) of mankind is set
From the very beginning, creation's purpose had everything to do with bringing about mercy.
10 For the mountains shall depart and the hills be removed, but my kindness shall not depart from thee, neither shall the covenant of my peace be removed, saith the Lord that hath mercy on thee. (3 Nephi 22:10; see also Isaiah 54:10)
God's mercy is more enduring than the mountains -- I might add any aspect of the creation itself. That is a powerful thought. A revelation to Joseph Smith goes into more detail:
48 And then shall the Lord set his foot upon this , and it shall cleave in twain, and the earth shall , and reel to and fro, and the also .
49 And the Lord shall utter his voice, and all the ends of the earth shall hear it; and the nations of the earth shall , and they that have shall see their .
50 And calamity shall cover the , and the scorner shall be consumed; and they that have watched for iniquity shall be hewn down and into the .
...
55 And shall be , that he shall have no place in the hearts of the children of men.
56 And at that day, when I shall come in my , shall the parable be fulfilled which I spake concerning the ten . 57 For they that are wise and have received the , and have taken the Holy Spirit for their , and have not been deceived—verily I say unto you, they shall not be hewn down and cast into the , but shall abide the day. (Doctrine & Covenants 45)
In order to "abide" the day of judgment, we must be changed as a result of the choices we make in mortality. When we walk the covenant path, we become new creatures, destined for a higher plane of existence. Everything and everyone else remains in the dust at the day of judgment.
Mercy comes through the Word
One final thought to wrap up this post:
If Adam and Eve could have made the plan "void" by partaking of the fruit prior to repenting, doesn't it follow that we do the same for ourselves if we reject the plan of happiness?