Romans 5:12-21 contrasts or distinguishes between the reign of death introduced by the sin of Adam, and the reign of life introduced by the atonement or repayment of Christ. The sequence is, first sin, then death. Now, the death which passed over humankind had its origin in Adam’s sin [as illustrated in Genesis 3:1-19]. Strictly speaking, there could be no individual sin till there was a law to be broken. But in the interval between Adam and Moses, i.e., before the institution of law [in Exodus 20:3-17], death prevailed over the world which was a proof that there was sin somewhere. The explanation is, that the sin in question was not the individual guilt of individual transgressors, but the single transgression of Adam from which all sin derives. [The Doctrine of The Original Sin].
Here, then, is the contrast. The single sin of the one man, Adam, brought death upon all humankind; BUT the single act of the ultimate Redeemer Jesus cleared away the offences—also for all humanity. Under the old law entered in to intensify the evil; but, in like manner, under the new, grace [God’s unmerited favour] has come in to enhance and multiply the benefit.
The treatment of the righteousness of God, as a Divine gift to sinners in Jesus Christ, is now complete. St. Paul’s main purpose is unmistakable in Romans 5:12-21. It brings out the grandeur and completeness of Christ’s work by contrast with the work (so to speak) of Adam. It regards the two as, in some real sense, paralleled and balanced but with Christ finally winning the battle.