The law of justice works in relationship to the other laws of God in the moral realm. In essence, the law of justice might be explained as follows: (1) every law has both a punishment and a blessing attached to it; (2) whenever the law is transgressed (broken), a punishment (or suffering) must be inflicted; (3) whenever a law is kept (obeyed), a blessing (or reward) must be given.
The law of justice requires that God must be a God of order and that he must be just and impartial. Because of the law of justice, God can make such statements as these: “I the Lord, am bound when ye do what I say; but when ye do not what I say, ye have no promise” (Doctrine and Covenants 82:10); “There is a law, irrevocably decreed in heaven before the foundations of this world, upon which all blessings are predicated—And when we obtain any blessing from God, it is by obedience to that law upon which it is predicated” (Doctrine and Covenants 130:20-21).
The law of mercy agrees entirely with the law of justice. However, the law of mercy introduces the possibility of vicarious payment of the laws that have been transgressed. In essence, the law of mercy might be paraphrased as follows: Whenever a law is transgressed (or broken), a payment (or suffering or atonement) must be made; however, the person who transgressed the law does not need to make payment if he will repent and if he can find someone else who is both able and willing to make payment. Note that the law of mercy insists that the demands of the law of justice be met fully. As Alma stated, … “justice exerciseth all his demands, and also mercy claimeth all which is her own; and thus, none but the truly penitent are saved. What, do ye suppose that mercy can rob justice? I say unto you, Nay; not one whit. If so, God would cease to be God.” (Alma 42:24-25.)
The law of justice made the atonement of Jesus Christ necessary. When Adam fell, he transgressed a law that had physical and spiritual death as its punishment. Thus the law of justice demanded payment (or atonement) for the broken (or transgressed) law.
The law of mercy made the atonement of Jesus Christ possible. In order for Jesus Christ to pay fully for the law Adam had transgressed, it was necessary that the Savior be both able and willing to make atonement. He was willing to make payment because of his great love for mankind, and he was able to make payment because he lived a sinless life and because he was actually, literally, biologically the Son of God in the flesh. Thus he had the power to atone for the spiritual and physical deaths introduced by the fall of Adam and Eve. Because of this atonement (or payment), he is rightfully referred to as the Savior and Redeemer of all mankind.
Every person benefits unconditionally from two major aspects of the atonement: (1) the resurrection, and (2) the full payment for the original transgression of Adam and Eve. However, as Mosiah indicates, there are also some conditional aspects of the atonement, and in order to benefit from these a person must repent of his sins. Otherwise, “mercy … could have claim” upon the person “no more forever,” for the law of mercy is made active in the life of a person only upon the conditions of repentance. (Mosiah 3:25-27.)