It is true that God has appointed that all mankind must die physically and spiritually, both of which are the result of the Fall of Adam. God has appointed that all must come to a judgment before Him. And it is given unto us to know good from evil. Given that these conditions have been appointed unto us by God, it makes sense that God would tell us of these conditions of which He has appointed. The following from Alma 12:27-37 not only tells us what He has appointed, but that He has made these conditions known unto us by sending messengers, first to Adam where His words were recorded, and later to others. Through these messengers He also made known unto us His Plan of Redemption through the atonement of His son.
"...it was appointed unto men that they must die; and after death, they must come to judgment, even that same judgment of which we have spoken, which is the end (27).
And after God had appointed that these things should come unto man, behold he saw that it was expedient that man should know concerning the things whereof he had appointed unto them (28). Therefore he sent angels to converse with them, who caused men to behold of his glory (29).
And they began from that time forth to call on his name; therefore God conversed with men, and made known unto them the plan of redemption which had been prepared from the foundation of the world; and this he made known unto them according to their faith and repentance and their holy works (30).
Wherefore, he gave commandments unto men, they having first transgressed the first commandments as to things which were temporal, and becoming as gods, knowing good from evil, placing themselves in a state to act, or being placed in a state to act according to their wills and pleasures, whether to do evil or to do good-- (31)
Therefore God gave unto them commandments, after having made known unto them the plan of redemption, that they should not do evil, the penalty thereof being a second death, which was an everlasting death as to things pertaining unto righteousness; for on such the plan of redemption could have no power, for the works of justice could not be destroyed, according to the supreme goodness of God (32).
But God did call on men, in the name of his Son, (this being the plan of redemption which was laid) saying: If ye will repent, and harden not your hearts, then will I have mercy upon you, through mine Only Begotten Son (33);
Therefore, whosoever repenteth, and hardeneth not his heart, he shall have claim on mercy through mine Only Begotten Son, unto a remission of his sins; and these shall enter into my rest (34).
And whosoever will harden his heart and will do iniquity, behold, I swear in my wrath that he shall not enter into my rest (35).
And now, my brethern, behold I say unto you, that if ye will harden your hearts ye shall not enter into the rest of the Lord; therefore your iniquity provoketh him that he sendeth down his wrath upon you as in the first provocation, yea, according to his word in the last provocation was well as the first, to the everlasting destruction of your souls; therefore, according to his word, unto the last death, as well as the first (36).
And now, my brethern, seeing we know these things, and they are true, let us repent, and harden not our hearts, that we provoke not the Lord our God to pull down his wrath upon us in these his second commandments which he has given unto us; but let us enter into the rest of God, which is prepared according to his word (37).
And in verse 1 of Alma Chapter 13, He makes is clear as to why He " ordained priests, after his holy order, which was after the order of his Son, to teach these things (Alma 12) unto the people" (emphasis added).