Response to Jeff regarding sin continued...
My understanding of individual sins, big and small, is that they are symptoms of a larger problem. That problem is that human beings have a sinful nature, which flows down to us from Adam and Eve, who ruptured humanity's relationship with God. And because we are all born into a family that is at emnity with God, we all initially begin in a state hostility with God. This hostile relationship leads human beings not accepting God's rightful authority over them, and consequently perpetually disobeying God.
Part of the point of the Sermon on the Mount was to say that the point isn't big sins or small sins (eating shrimp vs. murder), but rather that even those people who appear good on the surface are full of sinful thoughts and feelings, and this illustrates the way in which we are all out of line with God. This illustrates that we all have a sin nature, even if some of us hide it better than others. The point of the Sermon is that we are all far out of line with God's standard, perfection.
I do think Jeff pointed correctly to God's forgiveness as addressing our sins. However, I think it's important to note that forgiveness of our moral debt, our sin, requires that God take the loss. This is described in Psalm 130:8: "He himself will redeem Israel from all their sins." Christ is the fulfillment of this promise that God would redeem his people. So forgiveness is the answer, but Christ as savior, as sacrificial lamb on the cross, is the way that God accomplishes that forgiveness.