In answer to your question, one cannot improve on what God says through Paul on this exact topic (Romans 5:12-19):
"Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men[e] because all sinned— 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 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.
18 Therefore, as one trespass[f] led to condemnation for all men, so one act of righteousness[g] leads to justification and life for all men. 19 For as by the one man’s disobedience the many were made sinners, so by the one man’s obedience the many will be made righteous."
If that is not clear enough, here is how I explain it in “God’s Glorious Story” (pp. 82-84):
To question the historicity of these events of creation, including
the instantaneous and unique creation of Adam and Eve by God as the first
humans, is to challenge the very essence of Christianity. Why is that? Because
to do so completely undermines the biblical doctrine of justification of the
sinner by grace alone, through faith alone, in Christ alone (from R. Albert Mohler, Jr., “Adam and Eve: Clarifying Again What Is at Stake.”
Blog post from http://www.albertmohler.com/2011/08/22/false-start-thecontroversy-
over-adam-and-eve-heats-up/, August 22, 2011.) In the post,
Mohler references his article, “False Start? The Controversy Over Adam and
Eve Heats Up,” Monday, August 22, 2011.
Paul’s presentation of the doctrine of original sin and what God has
done about it in Romans 5 absolutely depends upon a historical Adam and
a literal interpretation of the account in Genesis. Everything Paul has to say
about justification of the sinner by faith depends on a literal interpretation
Paul writes, “Therefore, just as sin came into the world through one
man, and death through sin, and so death spread to all men because
all sinned” (Romans 5:12). He reinforces this in his first letter to the
Corinthians: “For as by a man came death, by a Man has come also the
resurrection of the dead. For as in Adam all die, so also in Christ shall
all be made alive” (1 Corinthians 15:21-22). Clearly Paul regarded both
the creation and fall of Adam as history, not allegory. In these verses, Paul
is explaining that when Adam and Eve sinned, it caused the whole human
race to be born “dead.” This means that the entire human race is born into
condemnation because of the sin of Adam (cf. Jeremiah 17:9; Romans 3:10;
Ephesians 2:1-3; 5:6; 1 John 5:19).
And the manner by which Paul describes how the death of one man—
Jesus Christ—could save so many is this: the death of one perfect Person, one
perfect Sacrifice, is sufficient to cover all the atonement necessary because all
that which requires atonement derives from an original disobedience. Paul explains this
in Romans 5:18-19: “Therefore, as one trespass led to condemnation
for all men, so one act of righteousness leads to justification and life
for all men. For as by the one man’s disobedience the many were
made sinners, so by the one Man’s obedience the many will be made
Here’s the point: because one sin had such a comprehensively
damning effect on the whole human race, one sacrifice can have an equally
comprehensive rescuing effect on the whole human race. That is why Christ
is called the “last Adam” (1 Corinthians 15:45).
But here’s the flip-side: if sin did not derive from an original disobedience
in an actual pair of original humans, then the sacrifice of Christ cannot have
obtained God’s desired effect of atoning for all sin. If there is somehow
sin in the world not deriving from Adam, then the Bible’s own logic would
conclude that such sin has not been redeemed by Christ. And if that is true,
then there is a deficiency in how that sin might be justified before God,
because that sin has not been properly addressed. That sin has not had
adequate atonement. That sin has not been covered by the “precious blood
of Christ” (cf. 1 Peter 1:19). That sin still needs some saving act other than
what Christ accomplished with His death and resurrection—the pinnacle
events in God’s glorious story.
The conclusion? The evolutionary concept that sin did not necessarily
derive from a historical Adam and Eve is completely inconsistent with the
biblical account of how God justifies the sinner. As such, for those who hold
to the authority of Scripture, it must be discarded.
I hope this helps. It really is crucial in understanding exactly why Christ’s sacrifice has the effect that it does in providing a way of forgiveness.