How should we properly understand Adam and Eve before the Fall? This is a tough question and almost everything hinges on it - how we understand grace, nature, salvation, merit, the incarnation of Christ, the Blessed Virgin Mary, and eternity.
What does it mean to be human?
First, when we speak of what it means to be "human" we must not look at our fallen state. Rather, we should examine "humanity" with regard to our prelapsarian (pre-fallen) state. We know that Adam and Eve were constituted in grace. Consequently, we cannot speak of them as purely natural or as being "homo naturalis". Further, they were given preternatural gifts that extended (praeter) their natural abilities. For example, they were granted immortality, their passions were properly integrated to their intellect and will, and their intellects were brighter and less encumbered by the distractions of our lower appetites (e.g. hunger, sexual desires).
Adam and Eve as in grace but not yet perfected in grace
Although they were in grace, they were not yet perfected in grace now were they fully glorified. In other words, they had not yet been admitted into the Beatific Vision. Allegedly, the presence of Satan and the test of the tree in the garden was a probationary arrangement. If Adam had succeeded, he and his bride would have been glorified and been admitted to the beatific vision of God's essence.
Grace doesn't only "correct sin"
We should be careful not to think of grace as something that is only necessary to "correct sin". This is a temptation among Calvinistic theology. These thinkers wrongly presume that the the pre-fallen state was "graceless". The Catholic Church teaches that Adam and Eve were constituted in grace prior to the Fall (cf. Council of Trent (Session V, 1). The Catholic Church teaches that Adam "fell from grace"; where as some Protestants teach "Adam did not fall from grace, because he wasn't sinful and was therefore not in a state of grace." This begs the question: If Adam "fell", then from what did he fall? It seems that the answer is that Adam fell from nature. This is the Catholic heresy of Baianism - that Adam did not possess grace before the fall and that Adam "fell from nature".
Why the false doctrine of Total Depravity depends on "falling from nature"
Once we see that Adam fell from grace and did not fall from his human nature, we come to understand that fallen humanity is not totally depraved as Calvinistic Synod of Dordt would have us to believe. If we fell away from our natures, then original sin is something much more tragic and pessimistic. It follows logically that we would no longer be in the image of God. However, if we fall from grace, we retain our human nature. We lack the gifts we once possessed, but we are not ontologically changed into something else. We don't have a "sin nature" as some Protestants call it. Instead, we have what the Catholic Church (and Rom 7:7-8; James 1:14-15) calls concupiscence - a disordered inward desire for that which is contrary to reason.
The correct understanding of Original Sin
Original Sin is then the privation or absence of sanctifying grace and the original righteousness once possessed by Adam and Eve. It is not a "sin nature". It is not "total depravity". According to Saint Augustine, evil and sin is not some "thing"; rather, it is the lack of righteousness or justice. Original Sin is the lack of Original Justice. We are born without the grace, gifts, and privileges of Adam and Eve. We are not born as "vipers in diapers" be we are born naked and empty with respect to God's grace.
How does this relate to salvation as proclaimed by the Catholic Church?
Salvation entails two states - grace and glory. We can be restored to grace in this life. This is called "regeneration" or being "born again". This grace is conferred in baptism as the "sacrament of faith". If sacramental baptism cannot be conferred (because of imminent death), the grace is conferred by desire so that God is never handcuffed on account of His own sacraments (which by the way would be ridiculous). Having been justified in this way, the Christian is a pilgrim and begins to grow in grace so that he or she becomes a Saint. This is process of sanctification - a process that is not unrelated to justification. Thus salvation entails faith and works.
Grace leads to Glory
The journey of sanctification leads to the final state of glory. Glory is the vision of God's divine essnce. This is our greatest happiness because only God can satisfy our every desire. We are glorified and made, as St. Thomas Aquinas calls it, deiform. This doesn't mean that we become gods, but it does mean that we become like God - as much as a created human can become like God.
Since those in grace continue to struggle in concupiscence, sin is a real obstruction to grace. Those who die in a state of grace but are unrepentant or who do not properly repent of their sins will undergo "remedial correction" in the afterlife. This is the state of fiery purification described by St. Paul in 1 Cor 3:15. It does not supplement the merits of Christ, but it is the means by which the merits of Christ transform us into the image of His own holiness.
To be continued.