Here is how John Wesley consistently defined sin:
By sin, I here understand outward sin, according to the plain, common acceptation of the word; an actual, voluntary transgression of the law; of the revealed, written law of God; of any commandment of God, acknowledged to be such at the time that it is transgressed. (“The Great Privilege of Those that are Born of God“)
The criticism I’ve heard of this definition is that it does not take account of unintentional or accidental sins. Wesley’s response to such questions was that those things might be considered wrongs we commit, but not properly sin. They do not damage our relationship with God.
The complication here is that the Old Testament clearly describes sacrifices for unknown sins. In the first covenant, there is such a thing as a sin committed in ignorance of the law. If such at thing is possible, then why would those no longer be considered sins under the covenant of Christ?
My thinking — and I do not pretend to be a brilliant thinker here — has to do with the once-for-all sufficiency of the sacrifice of Christ. In other words, on the cross Jesus covered by his blood all sins of ignorance and accident. Such things are still sins in the sense that they are actions that void the law of God, but they are not sins in that they have already been atoned for and without our conscious participation in them do not represent a deliberate turning away from God, for which we would need to repent. Under the new covenant, such sins of ignorance have been paid for in advance and therefore do not damage our relationship with God. Of course, all our actual sins have been paid for as well, but since these involve a deliberate act of will, we must engage in an act of will to redeem the promise already made. We must repent and seek forgiveness to mend the rupture in our relationship with God.
That is not as elegant as I would like, but it is my attempt to be faithful to the biblical witness.
What do you think? What have your read or heard that helps you work through such questions?