From Rom, 5:12, Judaism disappears, and Paul takes a wider view. He looks at the whole state of man in the light of the sin the human race before God. By one offence, Adam brought on the whole race, sin, death and alienation and exclusion from God. Because of grace and one righteous act man could receive justification of life. Not all were justified, any more than condemned, but the act had a universal bearing on the whole race. It is the same word as ‘unto’ all, in contrast with ‘upon’ all that believe, in Rom 3:22. Adam’s work bore on all, and so did Christ’s work too.
Verse 13-17 are a parenthesis. Sin was in the world from Adam to Moses. As there was no law yet, there could be no charges – there was no law forbidding them. Yet death reigned over those who had not sinned like Adam in breaking God’s actual commandment. as Adam did). Later a formal law was given under Moses. But between the two, where there was no formal law; there was sin and death; there universal ruin. When grace came in, it dealt with a multitude man’s offences in responsibility , so that those who receive abundance of grace and of the gift of righteousness would reign in life by Jesus Christ, who was in God’s counsels before the world began.
In v. 19 we have the positive efficiency or effect on those who were actually connected with these two heads (Adam and Christ). By the disobedience of Adam, many are constituted sinners; by the obedience of Christ, many are constituted righteous. This is not responsibility and imputation, but a state into which the many were brought – righteous before God. The great point here is that the Lord and Adam, by their act and conduct, bring those connected with them into their appropriate conditions.
What is important to see is that the state of men was the consequence of the conduct of the head, not the conduct of men being met by the head. The law came in that the offence might abound, so that those who contravened the law were disobedient. Sin had reigned unto death. Had righteousness reigned, because of sin, it must have led to condemnation.
Christ, by His obedience, demonstrated God’s grace. Because God is love and rich in mercy, grace reigned through righteousness. If it had been man’s righteousness, it would not have been grace; but because of His obedience, the many are constituted righteous, and grace reigns through righteousness unto eternal life. Stated so clearly, that is the ground and way of our salvation.
A simplified summary of part of the introduction to John Nelson Darby’s Exposition of the Epistle to the Romans