"Therefore, just as sin came into the world through one man and death through sin and so death spread to all humanity because all sinned..."*So Paul begins his thought in Romans 5:12 but it is a thought he doesn't seem to finish. That "just as" of Paul's sentence causes us to expect another half to this statement; a "so also". We might expect something like "just as sin came into the world through one man so also righteousness came into the world through one man." But we don't see that right away. Instead, it seems that right in the middle of his thought Paul realized that he needed to make some clarifications before even completing his analogy as if the analogy might have been too badly misunderstood and could not be recovered if he does not first clear up some things.
We'll get to those clarifications in a moment but it maybe helpful to first see the main comparison Paul is making. That is, Christ is like Adam in that both have done something that impacts all of humanity. Adam was the one who allowed sin to enter the world through his own disobedience to God. Notice the kind of picture that Paul's language portrays here, as if sin were this personified force that had been locked out of the creation but by Adam's disobedience the door was left open for sin to enter and bring death with it. Once sin and death entered the creation, they ran rampant and unrestrained. As Paul says, they reigned like kings over God's creation. We've already seen in previous sections of Romans that God gave the law to help combat this hellish reign on earth but the law proved ineffective in that all humanity continued to sin anyway. Now that sin has made its way into the world, its power was too great to be resisted.
We have to skip down to v. 18-21 to get the other half of the equation, the "so also." If sin can be allowed to enter the world through one man's disobedience, then it stands to reason that the reign of sin might also be defeated by one man's faithfulness. This is what Paul believes Jesus has done. The unique faithfulness of Jesus Christ has allowed a new power of righteousness to enter the world in order that humanity might be made righteous and that new righteousness has brought new life along with it. Just as one man brought sin and death to all so also can one man bring righteousness and life to all.
But it is not as if these were two equal powers, sin and righteousness, now warring within the creation. Paul says that what Christ did is already overtaking what Adam did. That is the clarification Paul makes in v. 15-17. If many died by Adam's sin, much more will many live because of Christ. If Adam's one sin brought condemnation, Christ's one life of faithfulness overcomes many sins. If death reigned through one man, much more will those who receive grace and righteousness reign in life. Adam is a type of Christ but what Christ has done is far greater. Jesus has not merely leveled the playing field between sin and righteousness. He has won the decisive battle against sin and death and they are now retreating before the powers of righteousness and life advancing in our world.
Of course, that retreat of sin and death before the powers of life and righteousness doesn't always seem so obvious in our world. Often it may appear things are moving in the other direction, even in our own lives. Although the decisive blow has been struck, these wannabe kings of sin and death do not easily give up the territory they have held for so long. This, Paul will say in the next chapter, is why we must continually submit ourselves as instruments to the cause of righteousness. But it is because of what Paul has proclaimed in this chapter that we know we are submitting ourselves to a winning cause; the faithfulness of Jesus has overcome the transgression of Adam.
*You may notice that in some translations of Romans 5:12, the final phrase reads “in whom all sinned.” That final phrase in Romans 5:12 reads "eph ho all sinned", that "eph ho" being the Greek phrase that is in question. These are fairly simple words in Greek; "eph" being a shortened version of a common Greek preposition typically meaning "in" or "on" and "ho" being a relative pronoun meaning "whom". So a very literal translation of this phrase would read "in whom all sinned." If this is the proper translation, then Paul would be saying that when Adam sinned everyone sinned. That is, the entire human race is implicated in Adam's sin and found guilty because of what he did. St. Augustine, a Bishop in the North African city of Hippo in the late 300's and probably the single most influential theologian in the history of the Church, understood the phrase this way and it was in his understanding of this verse that he saw the doctrine of original sin; the idea that because Adam sinned all of humanity is guilty. Indeed, if "in whom" is the proper translation of this verse then it would be difficult to understand it any other way. Prominent as Augustine was, his teaching of original sin has impacted generation after generation of the Church, even those who have never heard the name of Augustine.
However, there is debate about the meaning of this verse because "eph ho" can also be a conjunction meaning "because". This would change substantially how we would understand what Paul is saying here. If Paul meant to say "death came to all humanity because all sinned" then Paul is not saying everyone is guilty because of Adam's sin, as Augustine thought, but that all human beings are guilty because all human beings have, in fact, sinned. Either way, we are all guilty. The question is this: Are we guilty because of what Adam did or because what we have done? As I have hinted at in this post, I think the idea that is most consistent with the rest of Paul’s thought is to understand Paul as saying that Adam opened the door for the power of sin to enter the world and that humanity has been powerless to stand before this force with the result that we have all sinned. So all have sinned as a result of Adam’s sin because it allowed the power of sin to enter the world, not because everyone is guilty due to Adam’s single transgression.