Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, for all have sinned ... (Romans 5:12, though the whole passage is instructive)But Paul's point in this passage, overwhelmingly, is that Christ's goodness far surpasses Adam's badness, that Christ's righteousness eclipses Adam's unrighteousness, that "the gift is not like the trespass".
Paul develops the theme with the familiar Hebrew "how much more" style of logic: if a single act by a bad man can have bad consequences worldwide, how much more does the single gift of God have redeeming consequences worldwide.
Most Christian thought shows no real doubt about everyone's sinfulness. But whether Christ's grace reaches out to all, some express doubt. With Christ, many see "if" and "unless" and other limits to the grace that comes through Christ. The mental list of exceptions pile up for how the consequences of Christ's act aren't quite as wide-scoped as the consequences of Adam's act. In effect, we think "How much less" Christ accomplishes than Adam. But Paul's point was the opposite: How much more.
But the gift is not like the trespass. For if the many died by the trespass of the one man, how much more did God's grace and the gift that came by the grace of the one man, Jesus Christ, overflow to the many. (Romans 5:15)
We act as if the sin of Adam was greater than the righteousness of Christ, or as if the sin of man might have further reach than the grace of God. So next time someone discusses Romans 5 as if the main point is original sin, remember Paul's point in bringing up the original sin: to show how small a thing it was next to God's grace:
Consequently, just as the result of one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous. (Romans 5:18-19)