{Therefore} (dia touto). "For this reason." What reason?
Probably the argument made in verses  1-11 , assuming our
justification and urging exultant joy in Christ because of the
present reconciliation by Christ's death and the certainty of
future final salvation by his life. {As through one man} (h(9373)per
di' henos anthr(9370)ou). Paul begins a comparison between the
effects of Adam's sin and the effects of the redemptive work of
Christ, but he does not give the second member of the comparison.
Instead of that he discusses some problems about sin and death
and starts over again in verse  15 . The general point is plain
that the effects of Adam's sin are transmitted to his
descendants, though he does not say how it was done whether by
the natural or the federal headship of Adam. It is important to
note that Paul does not say that the whole race receives the full
benefit of Christ's atoning death, but only those who do. Christ
is the head of all believers as Adam is the head of the race. In
this sense Adam "is a figure of him that was to come." {Sin
entered into the world} (h(8820)hamartia eis ton kosmon eis(886c)then).
Personification of sin and represented as coming from the outside
into the world of humanity. Paul does not discuss the origin of
evil beyond this fact. There are some today who deny the fact of
sin at all and who call it merely "an error of mortal mind" (a
notion) while others regard it as merely an animal inheritance
devoid of ethical quality. {And so death passed unto all men}
(kai hout(9373) eis pantas anthr(9370)ous di(886c)then). Note use of
dierchomai rather than eiserchomai, just before, second
aorist active indicative in both instances. By "death" in  Ge
2:17  3:19  physical death is meant, but in verses  17,21 
eternal death is Paul's idea and that lurks constantly behind
physical death with Paul. {For that all sinned} (eph' h(9369) pantes
h(886d)arton). Constative (summary) aorist active indicative of
hamartan(935c), gathering up in this one tense the history of the
race (committed sin). The transmission from Adam became facts of
experience. In the old Greek eph' h(9369) usually meant "on
condition that," but "because" in N.T. (Robertson, _Grammar_, p.
963).