I just stumbled across this again. We were discussing this a while back via email and he told me that he’d be OK with me posting what he said there. I think its helpful:
I think the distinction between mortal and venial sin is important. As far as our justification is concerned, all sins are mortal, as Luther pointed out in the Heidelberg disputation. Imperfection is imperfection, no matter if it is small or larger, no matter if one is aware of it or not. As far as our sanctification is concerned, some sins are venial, some are mortal. Venial sins usually fall under the category of sins which occur not purposely and willfully, but because we are by nature defective. Sometimes we are aware of them, sometimes not. Here the confession of sins of two types in the Small Catechism: Those of which we are not aware, and those of which we are aware. (This distinction goes back to the Old Testament, and the placing of garments on the priest, and the offering of specific sacrifices for unknown sins.) So if we are aware of a sin, does that necessarily mean it is a mortal sin? Well, we can we be aware of sin within us. For example, when something causes us to become angry, and we realize we should not be angry about that thing, and yet we are. At that moment there may be an awareness of the New Man, that the Old Adam has caused such anger, and must simply live with it until that anger subsides. Thus to prevent further or greater sin, the New Man takes the Old Adam for a walk…Here we can talk about non-willful sins, sins which occur because by nature we are defective, and which are a result of the reactions of that defective nature to our situation (we are tired and hungry and so end up yelling at a child when such yelling was not really needed…). But what about “premeditated sin”, that is sin which we commit with the full knowledge that it is sin, and we should not commit it? Here we are working against the Holy Spirit, and have strayed into mortal sin territory. So the chief example in the Bible: David and Bathsheba. Certainly with all the time that lapses during the sequence of events which occurred there there must have been something more than venial sin occurring. We don’t know. What we do know is that as soon as David is confronted with his sin, he does not claim ignorance, that he should not have committed adultery or murdered, but he repents, asks for a clean heart, a right spirit, and that Spirit not be taken from him, that the joy of salvation be restored to him.
I had asked: Does true anthropology support the notion that some sins at the time they are committed are not mortal because its not “me” who is doing them, but sin (Rom. 7) – in other words they are not “willful” as Scripture defines willfull?
Perhaps [it] can be seen in this way: They are not mortal not because something besides me is committing them, or because I am not aware of them, but because we live by faith in Christ.