As sin came into the world through one man and death through sin. The ‘one man’ is Adam; the reference is to the story of ‘man’s first disobedience’ in Genesis 3. Cf. Wisdom 2:23–24,
God created man for incorruption,
and made him in the image of his own eternity,
but through the devil’s envy death entered the world,
and those who belong to his party experience it
The same point is made in the outcry of 2 Esdras 7:118: ‘O Adam, what have you done? For though it was you who sinned, the fall was not yours alone, but ours also who are your descendants.’ Ben Sira characteristically draws a misogynistic moral from the narrative: ‘From a woman sin had its beginning, and because of her we all die’ (Ecclus. 25:24; see p. 153). But none of these writers sees anything of the deeper significance in the fall of man which is now unfolded by Paul.
Death spread to all men because all men sinned. Does this mean that all have sinned in their personal lives (which is apparently the meaning of the words in 3:23) or that all sinned in Adam’s primal sin? In support of the latter it might be argued that human beings are mortal before they commit any sin, so that the mortality of the race is the result of the original racial sin. This seems to be implied by verse 14, where those who lived between Adam and Moses are said to have died even if they did not sin in the manner of ‘the transgression of Adam’. The construction, with the underlying thought, is paralleled in 2 Corinthians 5:14: ‘one has died for all; therefore all have died’—where, however, it is the racial implication of Christ’s death, not of Adam’s fall, that Paul has in view. It is not simply because Adam is the ancestor of mankind that all are said to have sinned in his sin (otherwise it might be argued that because Abraham believed God all his descendants were necessarily involved in his belief); it is because Adam is mankind.
For a defence of the other interpretation—that the reference is to all having sinned in their personal lives—see Cranfield, ad loc.: after a careful examination of various interpretations of the words ‘because all sinned’ he concludes that this interpretation is ‘most probable’.(C. E. B. Cranfield, ‘On Some of the Problems in the Interpretation of Romans 5:12’, SJT Supp 22 (1969), pp. 324–341.)
Paul does not conclude his sentence with a ‘so’ clause to match the ‘as’ clause at the beginning of verse 12 –
Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because wall sinned—
His mention of death as spreading to all humanity because of sin leads him to introduce the long parenthesis of verses 13–17;
13 for sin indeed was in the world before the law was given, but sin is not counted where there is no law. 14 Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.
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 djustification.
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.
after the parenthesis, instead of providing the principal clause for which the reader has been waiting, he repeats the ‘as’ clause of verse 12 in different words in verse 18 and follows up the new ‘as’ clause with a balancing ‘so’ clause. An apodosis in correlative terms to the protasis (the ‘as’ clause) of verse 12 would be worded more or less like this: ‘so through one man God’s way of righteousness was introduced, and life by righteousness.’
F. F. Bruce, Romans: An Introduction and Commentary, vol. 6, Tyndale New Testament Commentaries (Downers Grove, IL: InterVarsity Press, 1985), 133–134.