The precept which operates to save Men from their past sins is called justification. Paul revealed the principles of justification in great detail in his epistle tothe Romans, beginning with Romans 3:21-26, which reads:
But now the righteousness of God without the law [of Moses] is manifested, being witnessed by the law [of Moses] and the prophets; Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: For all have sinned, and come short of the glory of God; Being justified freely by his grace through the redemption that is in Christ Jesus: Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
The Pharisees, a Jewish sect to which Paul had belonged, believed that salvation came through obedience to the Law of moses. To contradict that misunderstanding, he explains in verse 21 thatthe righteousness of God exists apart from the Law of moses, but is demonstrated in that Law and the teachings of the prophets. Then, in verse 22, he tells the gentile Romans that the righteousness of God is attained through faith in Christ, and that both Jew and Gentile have the same access to it. Verse 23 explains why Man needs a Savior to attain the righteousness of God. Though all have sinned and thus fallen short of His righteousness, they can be "justified" as a gift which can be given because of the grace of Christ (verse 24). Verse 25 says that redemotion comes through Christ because His suffering and death constituted a publicly visible payment or "propitiation" for the sins that Men "previously committed." Thus, it is as to past sins, from which Men are powerless to save themselves, that the atonement applies!
As men deal with the present necessity of learning obedience to God, the distinction between their past sins and the sins they are now committing, or will commit in the future, is strategic. Present and future sins can and must be dealt with by Men, who cannot be allowed to think that they are given a free ride into Heaven by God's grace. Hence, Paul specifically notes that the atonement applies only to past sins.
In verse 26 Paul explains that, without the atonement, it would be unjust favortism, and a violation of God's holy and perfect character to forbear from punishing some, while others must recieve fullpunishment. Were it not that Christ voluntarily undertook to suffer the punishment earned by those who have faith in Him, justice would not allow God to pass over any Man's transgressions. through the atonement, God can be both "just and the justifier" of those who have faith in Christ.
Men cannot justify themselves (Job 9:20;Luke 10:29, 16:15). Justification is a judicial act performed by the judge of all the earth, and literally means "to declare right or righteous." C.I. Scofield correctly explained the nature of justification in these words:
Justification...may be defines as the judicial act of God whereby He justly declares and treats as righteous the one who believes in Jesus Christ. The justified believer has been declared by the Judge Himself (Rom 3:31) to have nothing laid to his charge (Rom 8:1, 31-34).
Paul rejoiced in this great boon as he continued his exposition in Romans 3:27-28:
Where is boasting then? It is excluded. by what law? of works? Nay: but by the law of faith. Therefore we conclude that a man is justified by faith without the deeds of the Law [of Moses].
This passage confirms the reality that Men are not justified by the rituals of the Law of Moses or anything else they can do. No matter whatthe Lord asks of them, nomatter how perfect their obedience may be throughout the rest of eternity, all Men would have to be condemned for their past sins if Christ had not paid the price for them. If there had been no atonement, all Men would have to serve the eternal sentence they deserve for the crimes they previously committed.
Justification is, in essence, the decree of God that a Man's past sins are forgiven (Acts 13:38-39; Rom 3:23-26; 4:7-8). In fact, the word "justify" is often used in lieu of "forgiveness of sins" (Luke 18:14; Acts 13:39; Rom 3:20; 1 Cor 6:11; Gal 2:17). This great boon, according to Paul, may be obtained only "by the law of faith" (verse 27; Isa 53:11; Acts 5:31; Eph 1:7; Col 1:14).