In the Mass readings for this, the first Sunday of Lent, the reading from St. Paul's Letter to the Romans read:
"Brothers and sisters:
Through one man sin entered the world,
and through sin, death,
and thus death came to all men, inasmuch as all sinned.
For if, by the transgression of the one,
death came to reign through that one,
how much more will those who receive the abundance of grace
and of the gift of justification
come to reign in life through the one Jesus Christ.
In conclusion, just as through one transgression
condemnation came upon all,
so, through one righteous act,
acquittal and life came to all.
For just as through the disobedience of the one man
the many were made sinners,
so, through the obedience of the one,
the many will be made righteous."
Now, in this reading we see a very important idea - original sin. St. Paul says that by Adam's sin, by his rebellious, prideful disobedience to God, he brought sin and death to all men. But he also says, "inasmuch as all sinned". Why give this specification? Because Adam did not make every person guilty in the same way as when we voluntarily sin. Rather, by Adam's sin, we lost the destiny of eternal life in Beatitude which God intended for every person when He created them. Adam did not merely become guilty - he forfeited the spiritual and physical sustenance and immortality afforded to him by living in the garden of Eden, the paradise of living in complete harmony with God's Will. Before his sin, even the world in which he lived was without the death, pain and chaos which sin procured.
Adam sinned and was subsequently cast out of Eden, out of harmony with the condition of direct sustenance from and harmony with God. The consequence of being outside this harmony was an environment of death. As God's Creation, nature did not cease to exist when Adam sinned, and it retained the inherent goodness and life God gave at its beginning. But as Genesis tells us, "(t)horns and thistles shall it bring forth to you" (Genesis 3:18 newadvent.com Bible). Our relationships with ourselves were perverted (Gen 3:8;16-17), our relationships with one another were distorted (Gen 3:12-13; 16), and our relationships with nature were distorted as evidenced above. By sin, we abandoned God and so abandoned the source of all love, truth and goodness, falling into the death, chaos, enmity and pride of a sinful world.
By Adam's sin, humanity lost its destined spiritual life and its physical perfection, now vulnerable to age and decay, attacks from nature and the bodily consequences of personal sins. Through Adam's lineage, all of us inherited this physical mortality, this environment of sin, the existential stain of the human heart which, with this original sin, always has a weakness and a preference for sin. Further, Adam's sin opened up the fixation and proclivity for sin that has been characteristic of humanity ever since Eden. Whenever we encounter sin and are given the option to choose it, tempted by the Serpent, we automatically feel an inclination towards it, an emotional weakness for it. The Serpent makes it seem appealing to us, gives us innumerable excuses and rationalizations to choose it (Gen 3:4-5), and even tempts us to encourage others to choose sin as well. After all, in his miserable Hell, Satan loves company.
To return to my earlier question - what did St. Paul mean when he specified, "inasmuch as all sinned"? Because of the weakness to sin I described, the heritage of Adam's rebellion, every single person with this taint of original sin has sinned throughout history, at least venially once in their life. While we still retain free will, we have no real chance of being completely sinless in our lives. Indeed, being omniscient, God planned His act of salvation through Christ with this fact in mind. But how does Christ relate to original sin?
St. Paul said, "just as through one transgression condemnation came upon all, so, through one righteous act, acquittal and life came to all." By Adam's sin, all but the Blessed Virgin and Christ Himself have been damaged from birth with this original sin, with the spiritual and physical death, and near-certainty of personal sin it involves. But God, in His infinite mercy and justice, gave us a way out - the only just punishment to remove original sin: death. But not just any death - only the death of the most innocent being could be a high enough sacrifice to repair the damage Adam caused, to remove original sin from all humanity. There is only one way for this to be achieved, only one being truly pure enough to completely remove original sin, truly omnipresent enough to count for all men for all time, and to ensure the eventuality of the salvation of the entire universe: Christ. Through His Passion and crucifixion, Christ paid the penalty all of us deserve, through His resurrection He opened up that river of living water for all (even the dead), and through the Sacraments He instituted, we can partake of this New Covenant, washing away our original sin, our personal sins and renewing the destiny of Beatitude for which we were made. So universal is His grace that even those before His coming who truly were without personal sin were removed of original sin by His sacrifice, effective in all time and place. (Hebrews 11)
However, just as Adam did not give people personal guilt but opened up sin and a proclivity for it, Christ did not immediately save all people and justify all their actions. If Christ had justified our sins without any work to prove our contrition and faith beforehand, He would be unjust and thus go against His own divinity and revelation. Rather, by the spiritual death and resurrection of baptism, dying to sin and rising to eternal life in Christ (Romans 6:23), we are forgiven of the human guilt and physical distance between us and God imposed by original sin, and through the fire of Pentecostal love in the Spirit, we are given a new inspiration towards faith and life, a bolstering strength based in confident hope of the promises of Christ (Titus 1:2).
While the weakness towards sin and the physical mortality of original sin remains even after baptism, Christ's sacrifice also opened up the avenue of mercy. Adam's sin opened up the awareness and preference for sin, but Christ's sacrifice opened up the channel of merciful grace, spiritual renewal and strength that is acquired through the sacraments and sacramentals (such as reading the Bible and praying the rosary). Adam chose sin and thus tainted all humanity with sin; but Christ, the new Adam, chose mercy, and so all are given the opportunity to live in the life of the light of God. And by the certain hope of the salvation Christ has assured for us, we are empowered to the freedom of overcoming our sinful chains and joyfully carrying our cross.
This redemptive suffering, repeating the sacrifice of Christ and bearing in ourselves the penalty of sin, even that which we did not commit, we are "(a)lways bearing about in our body the mortification of Jesus, that the life also of Jesus may be made manifest in our bodies." (2 Corinthians 4:10; also see Romans 8:13) Until the Final Judgment, when the entire universe is brought to salvation or damnation unto the fullness of Christ's redemptive sacrifice, the world will not entirely reflect His new life in the Spirit, but through redemptive suffering, charity and other means, we may conform the world and ourselves to God, healing wounds, giving ourselves in aid, service and protection to others, and forever worshipping God in His Divine Liturgy - Heaven on earth. (Revelation 22:17)