1. Elevation to the supernatural order
When God created the first human couple, he constituted them in a state of holiness and justice, and offered them the grace of truly sharing in his divine life (cf. CCC 374, 375). This is how Tradition and the Magisterium throughout the centuries have interpreted the description of paradise contained in Genesis. Theology calls this state supernatural elevation , since it indicates a free gift, unachievable by our natural powers alone. This gift is not a requirement of human nature, but it is congruent with the creation of man in the image and likeness of God. To understand this point correctly, several aspects should be borne in mind:
a) It is not correct to separate man's creation from elevation to the supernatural order. Creation is not “neutral" with regard to union with God, but is directed towards it. The Church has always taught that our end or purpose is supernatural (cf. DH 3005), since we were chosen in Christ before the foundation of the world, that we should be holy and blameless before him ( Eph 1:4). That means there has never been a state of “pure nature," since God offered his covenant of love to mankind from the very beginning.
b) Although the purpose for which mankind was created was friendship with God, Revelation teaches us that at the beginning of history the first human couple rebelled and rejected communion with their Creator. This is “original sin," also called the “fall," precisely because they had previously been elevated to intimacy with God. However, when they lost God's friendship, they were not reduced to nothingness, but continued to be human, created beings.
c) This teaches us that although it is not correct to imagine the divine plan as divided into airtight compartments (as if God first created a “complete" man and then “in addition" elevated him), we should distinguish different orders within a single divine intention.  Based on the fact that, by their sin, the first human pair lost some gifts but kept others, Christian tradition distinguishes between the supernatural order (the call to friendship with God, and the special gifts lost by sin) from the natural order (all that God endowed them with when he created them, and which remains in spite of sin). These are not two juxtaposed or independent orders, since in fact what is natural was inserted in and directed towards the supernatural from the very beginning. The supernatural perfects the natural without cancelling it out. At the same time they should be distinguished from one another, as the history of salvation shows that the gratuitousness of God's gift of grace and redemption is different from the gratuitousness of his gift of creation; the former is a far greater manifestation of God's love and mercy. 
d) It is difficult to describe the state of innocence that was lost by Adam and Eve,  about which there are few statements in Genesis (cf. Gen 1:26-31; 2:7-8, 15-25). This is why tradition generally characterises their state indirectly, inferring (from the consequences of the sin narrated in chapter 3 of Genesis) the gifts that our first parents enjoyed and that were meant to be transmitted to their descendents. Thus, besides the natural gifts that corresponded to their condition as creatures, they received supernatural gifts, i.e. sanctifying grace, the divinisation that this grace brings with it, and the ultimate call to the vision of God. Together with these, Christian tradition acknowledges in paradise the “preternatural gifts," that is, gifts that were not demanded by nature but are congruent with it. They perfected nature on a natural level and constituted a manifestation of grace. These were the gifts of immortality, exemption from pain (impassibility) and mastery over concupiscence (integrity) (cf. CCC, 376). 
2. Original Sin
With the story of man's transgression of the command not to eat fruit from the forbidden tree, Sacred Scripture teaches that, at the serpent's instigation (cf. Gen 3:1-13), at the beginning of history our first parents rebelled against God, disobeying him and giving way to the temptation of wanting “to be like gods." In consequence, they received God's punishment, losing in great part the gifts with which they had been endowed (cf. Gen 3:16-19), and they were expelled from paradise (cf. Gen 3:23). Christian tradition has interpreted this as the loss of the supernatural and preternatural gifts, and also damage done to human nature, although the latter is not corrupt in essence. As a result of their disobedience, of preferring themselves to God, the first human pair lost grace (cf. CCC 398-399), and also harmony with creation and within themselves; thus suffering and death made their entrance into history (cf. CCC, 399-400).
The first sin took the form of a temptation that was accepted, since behind the disobedience of Adam and Eve lies the voice of the serpent, who represents Satan, the fallen angel. Revelation speaks of a previous sin by Satan and other angels, who, although they had been created good, rejected God irrevocably. Ever since mankind's first sin, creation and history have remained under the evil influence of the father of lies and a murderer from the beginning ( Jn 8:44). Although his power is not infinite, and is very inferior to God's power, it truly causes serious injuries to each person and to society. The fact that God allows diabolical activity does not cease to be a mystery (cf. CCC 391-395).
The biblical narrative also contains God's promise of a redeemer (cf. Gen 3:15). Thus redemption throws light on the extent and the gravity of man's fall, showing us the wonder of the love of a God who does not abandon his creatures, but comes to meet mankind in Christ's work of salvation. “We must know Christ as the source of grace in order to know Adam as the source of sin" (CCC, 388). “For 'the mystery of lawlessness' (2 Thes 2:7) is clarified only in the light of the 'mystery of our religion' (1 Tim 3:16)" (CCC, 385).
The Church has always understood this episode as an historical event—even though it has been transmitted to us in language that is certainly symbolic (cf. CCC, 390). Traditionally (since St Augustine) this has been called “original sin," since it took place at mankind's origin. Sin entered the world as the result of the wrong use of freedom on the part of created beings (first the angels and then man). Moral evil, therefore, does not come from the structure of society or from our material being; nor, obviously, from God, nor from an immovable destiny. Christian realism makes us face up to our own responsibility: we can do evil as the result of our freedom, and the one responsible for it is no other than ourselves (cf. CCC, 387).
Throughout the course of history, the Church has formulated the dogma of original sin in contrast to exaggerated optimism and existential pessimism (cf. CCC, 406). Pelagius affirmed that man can do good by his natural strength alone, and that grace is merely an external aid, thus minimising the extent of Adam's sin and of Christ's redemption and reducing them to merely good or bad example respectively. Against this, the Council of Carthage (418), following St Augustine, taught the absolute priority of grace, since the human being has been wounded through sin (cf. DH 223, 227; cf. also the Council of Orange, in the year 529: DH 371-372). Against Luther, who maintained that through sin man is essentially corrupted in his nature, with his freedom cancelled out and sin in everything he does, the Council of Trent (1546) taught that Baptism truly cleanses us from original sin. However, the consequences of sin remain—among them concupiscence, which we should not identify, as Luther did, with sin itself. Each person is free in his or her actions, and, sustained by grace, can merit through good deeds (cf. DH 1511-1515).
At the heart of Luther's position, and also of some recent interpretations of Genesis chapter 3, there lies an inadequate understanding of the relationship between (1) nature and history, (2) the psychological-existential plane and the ontological plane, and (3) what is individual and what is collective.
(1) Although there are some mythical elements in Genesis (understanding the concept of “myth" in its best sense, i.e. a narrative that gives origin to and therefore lies at the basis of later history), it would be wrong to interpret the narrative of the fall as a symbolic explanation of mankind's original sinful condition. Such an interpretation would change a historical event into something that is “part of man's nature," making a myth of it and therefore making it inevitable. Paradoxically, if the sense of guilt led us to acknowledge ourselves as “naturally" sinners, it would lead to a reduction or mitigation of our personal responsibility for sin, since we would be unable to avoid such a spontaneous tendency. Instead, it is true to say that sin belongs to man's historical condition, and not to his original nature.
(2) Since some consequences of sin remain after Baptism, Christians may strongly experience a tendency towards evil, and feel they are profoundly sinful, as is seen in the lives of the saints. However, this existential perspective is not the only one, nor indeed the most fundamental one, since Baptism really does cleanse away original sin and make us children of God (cf. CCC, 405). Ontologically, Christians in a state of grace are truly just in God's eyes. Luther radicalised the existential perspective, understanding the whole of reality from this viewpoint, and seeing it as ontologically marked by sin.
(3) The third point leads to the question of the transmission of original sin, “a mystery that we cannot fully understand" (CCC, 404). The Bible teaches that our first parents transmitted sin to the whole of mankind. The chapters in Genesis that come after the fall (cf. Gen 4-11; cf. CCC, 401) narrate the progressive corruption of the human race. Establishing a parallel between Adam and Christ, St. Paul says: For as by one man's disobedience many were made sinners, so by one man's [Christ's] obedience many will be made righteous ( Rom 5:19). This parallelism helps us to understand correctly the interpretation that is generally given to the term adamáh as a collective noun: just as Christ is only one and at the same time is head of the Church, so Adam is only one and at the same time head of mankind.  “By this 'unity of the human race' all men are implicated in Adam's sin, as all are implicated in Christ's justice" (CCC, 404).
The Church understands the original sin of our first parents and the sin that has been inherited by mankind by way of analogy. “Adam and Eve committed a personal sin but this sin . . . will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called 'sin' only in an analogical sense: it is a sin 'contracted' and not 'committed'—a state and not an act" (CCC, 404). Thus, “although it is proper to each individual, original sin does not have the character of a personal fault in any of Adam's descendants" (CCC, 405). 
Some people find it difficult to accept the idea of an inherited sin,  especially if they have an individualistic view of the person and of freedom. What did I have to do with Adam's sin? Why do I have to pay the penalty for someone else's sin? These questions show a lack of understanding about the real solidarity that exists among all human beings as created by God. Paradoxically, this lack of understanding can actually be seen as a manifestation of the sin transmitted to each individual. In other words, original sin clouds our understanding of the deep fraternity of the human race that makes its transmission possible.
The Catechism of the Catholic Church teaches that, faced with the lamentable consequences of sin and their universal spread, we can ask ourselves, “But why did God not prevent the first man from sinning? St Leo the Great responds, 'Christ's inexpressible grace gave us blessings better than those the demon's envy had taken away' ( Sermon, 73, 4). And St. Thomas Aquinas wrote, 'There is nothing to prevent human nature's being raised up to something greater, even after sin; God permits evil to draw forth some greater good. Thus St. Paul says, Where sin increased, grace abounded all the more ( Rom 5:8); and the Exultet sings, O happy fault . . . which gained for us so great a Redeemer ' ( Summa Theologiae, III, 1, 2, ad 3)" (CCC, 412).
3. Some practical consequences
The main practical consequences of the doctrine of our elevation to the supernatural order and original sin is the realism that should guide the lives of Christians, who are aware of both the greatness of being children of God and the wretchedness of their sinful condition. This realism:
a) prevents both naïve optimism and hopeless pessimism; it “provides lucid discernment of man's situation and activity in the world . . . Ignorance of the fact that man has a wounded nature inclined to evil gives rise to serious errors in the areas of education, politics, social action and morals" (CCC, 407);
b) gives serene trust in God, our merciful Creator and Father, who does not abandon those he has created, but always forgives, and leads everything to goodness, even in the midst of adversities. “Repeat: ' omnia in bonum, ' everything that happens, 'everything that happens to me' is for my good… Therefore the right conclusion is to accept, as a joyful reality, what seems so hard to you"; 
c) gives rise to an attitude of profound humility, which leads us to acknowledge our own sins without surprise, and to be sorry for them because they are offences against God, rather than because they are defect in ourselves;
d) helps us to distinguish what is proper to human nature as such, from what is a consequence of the wound of sin in our nature. After sin, not everything we experience as spontaneous is good. Human life entails a battle: we have to struggle in order to behave in a way that is both human and Christian (cf. CCC, 409). “The entire tradition of the Church has described Christians as milites Christi: soldiers of Christ, soldiers who bring serenity to others while continually fighting against their own bad inclinations".  Christians who struggle to avoid sin do not lose anything that makes life good and beautiful. There is a fairly widespread, but false, idea that it is necessary for people to do evil in order to experience their own autonomy, since a life without sin would be basically boring. In contrast, we see the figure of Mary, conceived immaculate, who shows us that a life completely dedicated to God, far from being boring, becomes an adventure filled with infinite surprises. 
Catechism of the Catholic Church, 374-421
Compendium of the Catechism of the Catholic Church, 72-78
John Paul II, God, Father and Creator. A Catechisis on the Creed (vol.1) , Boston: Pauline Books and Media1996, pp. 216 ff.
Denziger-Schönmetzer (DH; DS), Enchiridion Symbolorum , 36th ed (Herder 1976), nos. 222-231; 370-395; 1510-1516; 4313
John Paul II, Memory and Identity , London: Weidenfeld & Nicolson, 2005
Benedict XVI, Homily on the Solemnity of the Immaculate Conception, 8 December 2005 http://www.vatican.va/holy_father/benedict_xvi/homilies/2005/documents/hf_ben-xvi_hom_20051208_anniv-vat-council_en.html
Joseph Ratzinger, In the Beginning: a Catholic understanding of Creation and the Fall , London: T & T Clark, 1995
 The Council of Trent does not say that man was created in grace, but constituted, precisely so as to avoid confusion between nature and grace (cf. DH 1511)
 The theological hypothesis of “pure nature" was coined precisely to emphasise the complete gratuitousness of the gift of grace with respect to creation. Not because such a state existed historically, but because in theory it could have done so, although in fact it never did. This doctrine was upheld against Baius, one of whose theories (which was condemned) was that “the integrity of the first creation was not an unmerited exaltation of human nature, but a condition natural to it" (DH 1926).
 This difficulty is more acute today owing to the influence of a basically evolutionist view of the totality of the human being. In this type of approach, reality evolves constantly from the smaller to the greater, while Revelation teaches us that at the beginning of history there was a fall from a higher to a lower state. This does not mean that there could not have existed a process of “hominisation," which we have to distinguish from “humanisation".
 As to immortality, we have to understand this with St Augustine not as being unable to die ( non posse mori ), but as the possibility of not dying ( posse non mori ). It is licit to interpret it as a situation in which the transition to a definitive state was not experienced through the trauma of death that man suffers because of sin. Since suffering is a sign and anticipation of death, immortality in some way brought with it an absence of pain. At the same time, this presupposed a state of integrity, by which man ruled his passions without difficulty. Traditionally a fourth gift is added, that of knowledge, proportionate to the state in which they found themselves.
 This is the main reason why the Church has always read the account of the fall from the point of view of monogenism (the descent of the human race from a single couple). The opposite hypothesis, polygenism, seemed to impose itself as scientific (and also exegetic) fact for a few years, but today at a scientific level, biological descent from a single pair (monophyletism) seems more plausible. From the standpoint of the faith, polygenism is problematic, since it is not possible to see how this can be reconciled with Revelation about original sin (cf. Pius XII, Encyclical Humani Generis, DH 3897), although it is a matter about which there is still room for inquiry and reflection.
 In this sense, a distinction has traditionally been made between the originating original sin (the personal sin committed by our first parents) and the originated original sin (the state of sin in which their descendants are born).
 Cf. John Paul II, General Audience , 24 September, 1986, 1
 St. Josemaría, Furrow, 127; cf. Rom 8:28
 St. Josemaría, Christ is Passing By, 74
 Cf. Benedict XVI, Homily , 8 December 2005.