It comes as a surprise to some that there are gradations of sin according to the Bible. There are more serious and less serious sins, which is the basis of the later distinction between mortal and venial sins. It comes as even more of a surprise to many that there is no atonement in the OT for certain kinds of sins— the so-called ‘sins with a high hand’ which refers to premeditated sins, like the premeditated adultery with Bathsheba of King David. This is why he throws himself on the mercy of God in Ps. 51. There was not sacrifice he could offer to atone for such a heinous sin. Acts 13.38-39 puts the matter succinctly as Paul is preaching about the benefits of the atoning death of Jesus “through this man forgiveness of sins is proclaimed to you; by this Jesus everyone who believes is set free from all those sins from which one could not be freed by the law of Moses.” Besides, there was a limited warranty to the OT sacrifices. If one committed the same sins again, one had to make the same sacrifices again. And even Yom Kippur, the big day of atonement, only cover sins of the past, not future sins. This is why the author of Hebrews calls the atonement of Christ better, because it is ‘once for all sins, for all persons, for all time’ and makes unnecessary any more literal sacrifices. It also makes unnecessary any class of priests, who’s main job it is to offer sacrifices. There is no priesthood in the NT in the form of clergy, only the priesthood of all believers offering themselves up to God as living sacrifices, and the high priesthood of Christ in heaven. That’s it.