I think it’s a bit confusing to say that our original-nature is given, while sin or ignorance is an accretion. Every sort of Buddhist account of “the fall” I have known leaves precisely what separates original from fallen nature a bit obscure. What is Original Sin and Original Enlightenment.
The original sin was not an act by Adam or Eve anymore than it was an act by you or me. God’s very creation of Adam was that sin, and not because he was destined to along with Eve disobey God. That Adam and Eve were created separate from God and given the opportunity to subsist forever in this separation through the Tree of Life, is God’s own self-sundering.
Adam and Eve partake of the Knowledge of Good and Evil, which prefigures the Law, and throw themselves into sin in the course of ultimately transcending it for Him, but not apart from Him. This is why Zizek is right to see Christ’s rising in Adam’s falling, and why Paul calls Adam (in Romans 5:14) “a type of the one to come” (i.e. Christ).
A great book on sin is the first half of “Lying: A Augustinian Theology of Duplicity” by Paul Griffith. Pair that then with Karl Barth’s “Adam and Christ: Man and Humanity in Romans 5,” which is basically about the samsara/nirvana relationship.
I draw on Karl Barth here, but this is why I think he’s useful for working with the Buddhist terms: our sinfulness, our ignorance of our original nature and the active withdrawal from it (think of Augustine’s shtick about how as a boy he stole that pear just to steal it, just because it was wrong – he didn’t even like pears), is given along with that original nature – not as a later stain upon it.
If you want to use a term like “openness” (Heidegger’s Lichtung) to convey this original-nature, our openness is from the beginning an openness to that very ignorance of it. What is sinful is not apart from that original nature (samsara is nirvana), but sinfulness as sinfulness, ignorance in-itself is not self-sustaining either. It’s temporal; it arises and it ceases. In a sense, it doesn’t even arise /from/ original nature (lending itself to the idea that original-nature is temporally prior to sin or ignorance), but exists from the very beginning with it – co-dependently arising. Co-dependently-arising, our ignorance of our original nature ceases while our original nature remains.
This is what Dogen is getting at when he says in his Genjokoan that
“To study the buddha way is to study the self. To study the self is to forget the self. To forget the self is to be actualized by myriad things. When actualized by myriad things, your body and mind as well as the bodies and minds of others drop away. No trace of realization remains, and this no-trace continues endlessly.”
To forget the self is tantamount to taking that Adamic fall into ignorance and sinful-nature, but with the wholesome intention of realizing the original-nature heretofore under erasure. Myriad things are not-self (anatta), and the dropping away of body and mind is the dropping away of original sin. Dogen doesn’t just say it happens either, that it’s already there. Instead, it’s a moment in a process. The “no trace of realization [that] remains” is our original-nature, but it is not original nature “before” sin.
The weird idea some Christians have is that our sinfulness (or ignorance of God) is our original nature, which is to say that God created only to condemn us. To get us back on the path, whether we’re getting lost in the first Canto of Dante’s Divine Comedy or stumbling in an Ox-Herding painting, we’ve got to let go of sin, but we have to engage it in order to do that, just like we have to pass through ignorance of our original nature to realize it.
It would then seem that some Soto Zennists or otherwise proponents of shikantaza coupled with a one-sided view of “Original Enlightenment” are latter day Pelagians.