The Law as Stimulus to Sin
In Romans 5:12–14, Paul explicitly discusses Adam and draws a distinction between his sin and the sin of all others from Adam to Moses:
Therefore as sin came into the world through one man and death through sin, and so death spread to all men because all men sinned—sin indeed was in the world before the law was given, but sin is not counted where there is no law. Yet death reigned from Adam to Moses, even over those whose sins were not like the transgression of Adam, who was a type of the one who was to come.
Although the wording is somewhat confusing, I think that very important points can nonetheless be derived from this passage. Paul is making a distinction between “sin” on the one hand and “transgression” on the other. Adam's transgression was correctly accounted to him, because he had been given a law: the law, of course, forbidding him to eat of the tree of knowledge. Sin is separate from the Law. It is not caused by the Law; all men sin and the (natural?) consequence is death, even for those for whom sin cannot be accounted, because their “sins are not [accountable because they are not] like the transgression of Adam.” Adam ends up here being prototypical of two human groups: those who have the Law and thereby are subject to have their transgressions accounted (Jews) and those who are affected by unaccountable sin but nevertheless die as a result of it (gentiles). “Adam's sin was παράβασις obviously = ‘sin accounted’ since it was an act of disobedience to what he knew to be a command of God” (Dunn 1988, 276). Paul is at pains that we realize that even without accounting, sin itself nevertheless results in death, so that even those who have not sinned as Adam did—that is, even those who do not know the Law—are in exactly the same situation as those who know the Law. Paul's overall theme in Romans that Jews and non-Jews are in exactly the same situation is thus well supported by this argument. Paul is further counteracting, however, a Jewish argument or attitude which we have already seen him critiquing in Romans 2, namely, the attitude that having the Law provides some sort of immunity to sin or redemption from sin. This is the source of his assertion here and below that having the Law makes sin greater, not lesser. “God's purpose for the law was not to distinguish Jewish righteous from gentile sinners, but to make Israel more conscious of its solidarity in sin with the rest of Adam's offspring” (Dunn 1988, 286).
The only way to understand verse 20, “Law came in to increase the trespass,” is in context, in reference to Adam—one man's trespass—so Adam is clearly here the type of the Jew, human being under the Law. The content of this verse is interpretable in two ways, neither of them, at any rate, nearly so antithetical to rabbinic theologoumena as the Reformation tradition would have it. Either the knowledge of that which is forbidden increases culpability, or having the knowledge of that which is forbidden increases the desire to sin. Either way, the point is that Jews cannot claim any privilege, because they have the Law. Having the Law makes their salvation more difficult, not easier. Paul is fighting against a Jewish theology—held by some, not all, first-century Jews—which argues that just having the Law provides a privileged place in salvation for the Jews.