Romans 5:15–17 (ESV): 15 But the free gift is not like the trespass. For if many died through one man’s trespass, much more have the grace of God and the free gift by the grace of that one man Jesus Christ abounded for many. 16 And the free gift is not like the result of that one man’s sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brought justification. 17 For if, because of one man’s trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ.
In verses 12-14, Paul established the analogy of the two Adams by comparing their likenesses, but here Paul begins to contrast the the two to demonstrate the superiority of both the quality and the degree of the latter.
There is the free gift that Paul has already established in the argument leading up to this passage. The free gift is the righteousness of God given to those who, like Abraham, have faith. The trespass is the disobedience of Adam in the garden that plunged humanity into its depraved stated, as Paul demonstrated in chapters 1 and 2. These two are to be held in contrast.
The trespass of the first Adam brought death to many (euphemism for all mankind). The obedience of the Second Adam (that one man Jesus Christ) brought grace (quality) to the many but in abundance (degree).
The judgment caused by the first Adam’s one trespass brought condemnation while the obedience of the second Adam brought justification (though many others sinned after the manner of Adam).
Therefore, if death reigned as a result of the trespass of the first Adam, we can be sure that those who receive the abundance of grace and the free gift of righteousness given through the one man Jesus Christ will now reign in life.
In sum the first Adam trespassed and brought sin into the world and by sin brought condemnation, and ultimately death, for all humanity. The second Adam brought grace and righteousness and by that gift brought justification and life to all who believe.