The primordial sin of Adam is believed by Catholics to have been
inordinate love of self; or rather an attempt by the first humans to become gods on our own terms by setting ourselves and our self-centered desires up as the be-all-and-end-all of existence - to the exclusion of the Creator, our fellow human beings and creation itself of which we were meant to be the stewards on behalf of God.
According to Catholic thought, this inordinate love of
self is the cause of every
sin. Thus the 17th century Roman Catholic mystic Angelus Silesius wrote:
"Naught more than the words "mine and thine"—
(and mark this lesson well)—
Can plunge thee bodily into the jaws of hell
Believing that you yourself are so smart
and understand it all
condemns you to ignore
your ignorance -
this is the meaning of the Fall
Love is like death -
it kills the self-willed Me
it breaks its strangehold
And sets the Spirit free"
Adam and Eve lost that primeval "ignorance" or "innocence" because of inordinate, exclusive self-love when they ate of the tree of the knowledge of good and evil. Man started to think he could do as he pleased - hurt other human beings, ravage his own body, destroy nature - and the words, "mine" and "me" came into our lexicon - along with division of property, tribe, culture etc.
This decision to choose the 'self' and become inordinately attached too it arose from this primeval loss of original innocence. The tree of knowledge of good and evil is a metaphorical description of this episode, which we believe to have been an
actual cognitive development or event in human evolution - albeit not occurring as 'mythically' described (i.e. with talking snakes!). This 'tendency' towards inordinate love of self then passed down ancestrally to all human descendants, in the Christian theological understanding, as an inherited trait/condition.
It required God Himself to become man to redeem us and restore our original purity - the primordial balance between individual man, his Creator, his fellow humans and creation as a whole. In fact it actually requires an entirely "new creation" after death because we messed up so damn badly.
Nothing can liberate us more from the "strange-hold" of the "self-willed Me" in which Adam's sin imprisoned us than the redeeming, sacrificial love of God's Only Son Jesus Christ on the cross - according to our beliefs. For in our understanding, the sacrifice of Christ is the only entirely selfless act in the whole course of human history - the perfect, sinless, wholly innocent one freely willing to die for the sins of the multitude.
Because of this, only by participating in Jesus' sacrificial death and rebirth through baptism and thus sharing in His Sinless Divinity through his perfect humanity can human beings become what they were intended to be in the beginning and become free from the burden of sin - of the "self-willed Me".
Hence why Saint Paul wrote in the Letter to the Romans (chapter 6):