Why is the doctrine of the imputation of Adam's sin important?

From Gospel Translations

Aren't our own individual sins enough to condemn us? Why, then, does it matter whether we believe that Adam's sin is imputed to us as the ultimate basis of our condemnation? John Piper writes:

Now someone might say, why does this matter? Doesn't Romans teach in 3:23 that "all have sinned and fall short of the glory of God" individually? And doesn't Romans 6:23 teach that the "wages of sin is death"? And so if our judgment and condemnation are what the sins we do every day deserve, why does it matter if you can find a deeper cause of our guilt and death and condemnation—namely our union with Adam in his sinning at the beginning? ("Adam, Christ, and Justification: Part II")

There are at least three reasons why this doctrine is very important.

Crucial for Grasping Justification in Romans 5:12-21
First, it is crucial to understanding Paul's teaching on justification in Romans 5:12-21. Piper writes:

What's at stake here is the whole comparison between Christ and Adam. If we don't understand "because all sinned" in 5:12 as "because all sinned in Adam," the entire comparison between Christ and Adam will be distorted and we won't see the greatness of justification by grace through faith for what it really is.

Let me try to illustrate what's at stake. If you say, "Through one man sin and death entered the world and death spread to everybody because all sinned individually," then the comparison with the work of Jesus could be, "So also through one man, Jesus Christ, righteousness and life entered the world and life spread to all because all individually did acts of righteousness." In other words, justification would not be God's imputing Christ's righteousness to us, but our performing individual acts of righteousness with Christ's help and then being counted righteous on that basis. When Paul saw that as a possible misunderstanding of what he said, he stopped to clarify.

But what does it say about the work of Christ, if we take the words, "because all sinned" to mean "because all sinned in Adam"? Then it would go like this: "Just as through one man sin and death entered the world and death spread to everybody because all sinned in Adam and his sin was imputed to them, so also through one man Jesus Christ, righteousness entered the world and life through righteousness, and life spread to all who are in Christ because his righteousness is imputed to them." That is the glory of justification by grace through faith. The basis of our vindication and acceptance before God is not our righteous deeds, but Christ's righteousness imputed to us. But this would be all distorted if the words "because all sinned" at the end of verse 12 meant "because all sinned individually," and not because all sinned in Adam and his sin was imputed to us.

The parallel Paul wants us to see and rejoice in is that

                 just as Adam's sin is imputed to us because we were in him,
                 so Christ's righteousness is imputed to us because we are in him.

One of the best reasons for thinking this is what Paul meant is to look at verse 18 where he really does complete the comparison he started here. "So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men." In Adam we all were condemned; in Christ we all are justified. Adam's transgression was imputed to us; and Christ's righteousness is imputed to us (see 1 Corinthians 15:22).

But all that would be lost if at the end of verse 12 the words "because all sinned" referred to individual sins and not to our sinning in Adam. ("Adam, Christ, and Justification: Part II")

Highlights the Global Significance of Christ
Second, the doctrine of the imputation of Adam's sin highlights the global significance and universality of Christ. Piper writes:

If Adam is the father of all human beings, and if the fundamental problem with all human beings is found in how we are related to Adam and what happened to us when Adam sinned, then everybody in the world, no matter when or where or who—whatever tribe or language or culture or ethnic identity—everybody has the same fundamental problem. And this means that if Jesus Christ is not just a Jew who died as a Jewish sacrifice for sins, but is also the "last Adam" or the "second man" (as Paul calls him in 1 Corinthians 15:45, 47), who provides a righteousness better than what we lost in Adam, then Jesus is no tribal God, or limited, local Savior. He is the one and only remedy for the divine judgment of condemnation that rests on every human soul. Which means he is a great Savior able to save persons from all times and all places and all peoples. (Ibid)

Highlights the Global Significance of the Doctrine of Justification
Third, the connection between justification and the imputation of Adam's sin means that the doctrine of justification is not simply a Western doctrine. Piper writes:

Now let's drive this home for our missionaries and for all our evangelism here at home. Do not think that the doctrine of justification by grace, based on the imputation of the obedience of Christ through faith apart from works, is a mere concoction of a western European worldview that got off the ground with the guilty conscience of a monk named Martin Luther. That's not true. It can't be true, because it is the historical remedy in the person of Jesus Christ for the historical damage in the person of everybody's first ancestor.

The doctrine of justification by grace through faith cannot be replaced by a redemptive analogy. If Paul had merely said for example, "Sin is like drowning in the ocean, and salvation is like being pulled out of the water into a boat by a strong man," then you might go to a people group somewhere far from oceans and boats and say, "Sin is like sinking in quicksand and salvation is like being pulled onto a firm rock by a strong man." That's fine. But you can't do that with this doctrine of justification - not now, not after Romans 5:12-21.

Why not? Because now Paul has connected it with Adam. And Adam is the historical ancestor of every people group on the face of the earth. This is not a myth; it's not an analogy; it's not an illustration. It is historical fact. Adam, the first human being, sinned and in him all human beings sinned, and all died and all are condemned. And the remedy for that is another historical Person - the God-man, Jesus Christ, who came in space and time to undo what Adam did. He trusted and obeyed God perfectly, so that all who are in him by faith have that obedience imputed to them and become right with God forever. ("Adam, Christ, and Justification: Part I")

Further Resources

John Piper, "Adam, Christ, and Justification: Part I"

John Piper, Adam, Christ, and Justification: Part II

John Piper, "Adam, Christ, and Justification: Part V"