12 Therefore as sin came into the world through one man and death through sin, and so death spread to all men because all men sinned – 13 sin indeed was in the world before the law was given, but sin is not counted where there is no law. (Rom 5:12-13)
This text is the usual basic evidence of original sin. The first human, Adam, sinned and somehow we all inherited the guilt of that sin. We all know, deep inside, that such an idea is unjust - someone I never met did something wrong and I must be punished for it. Seems unfair; how to reconcile with God's justice and benevolence? The problem lies in our misunderstanding of original sin. It is not a particular sin (or guilt of such a sin) that we ontologically inherited; it is a phrase used to describe the condition of humanity. We refer to this condition euphemistically by the first sin ever.
A very nice explanation can be found in a work by Pope Benedict XVI (before he was pope):
The account (of Genesis 3) tells us that sin begets sin, and that therefore all the sins of history are interlinked. Theology refers to this state of affairs by the certainly misleading and imprecise term “original sin.” What does this mean?... Human beings have their selves not in themselves but also outside of themselves: they live in those whom they love and in those who love them and to whom they are “present.” Human beings are relational, and they possess their lives – themselves – only by way of relationship. I alone am not myself, but only in and with you am I myself. To be truly a human being means to be related in love, to be of and for. But sin means the damaging or destruction of relationality. Sin is a rejection of relationality because it wants to make the human being a god. Sin is loss of relationship, disturbance of relationship, and therefore it is not restricted to the individual. When I destroy a relationship, then this event – sin – touches the other person involved in the relationship. Consequently, sin is always an offense that touches others, that alters the world and damages it. To the extent that this is true, when the network of human relationships is damaged from the very beginning, then every human being enters a world that is marked by relational damage. At the very moment that a person begins human existence, which is a good, he or she is confronted by a sin-damaged world. Each of us enters into a situation in which relationality has been hurt. Consequently each person is, from the very start, damaged in relationships and does not engage in them as he or she ought. Sin pursues the human being, and he or she capitulates to it. (“In the Beginning,” Joseph Ratzinger, Wm . B. Eerdmans Pub. Co., Grand Rapids, MI, 1995, pp 72-3)
A clear truism is stated here: Humans are relational and we do not live for ourselves. We live for others. Our faith offers a way to destroy the ‘self-orientation’ that is the normal mindset of each person, and replace it with an ‘other-oriented’ tendency. God is, perhaps by definition, a pure state of other-orientation, the three Persons of the Trinity enjoying an infinite love for one another. We are called to participate in that divine Family and experience the infinite happiness that such an other-oriented life offers. This ‘way’ is none other than the life of grace, the sacramental grace given us by Christ.
20 But where sin increased, grace abounded all the more, 21 so that, as sin reigned in death, grace also might reign through righteousness to eternal life through Jesus Christ our Lord. (Rom 5:20-21)