Presumptuous sins are sins of undue liberty, of going beyond the bounds. They are willful, understood, and so directly culpable. It’s still breaking the law even when you don’t know the law, but with presumptuous sins we make eye contact with the judge to make sure he’s watching.
David was especially concerned about the enslaving nature of presumptuous sins. When we sin knowing full well what our will is doing, we actually give our wills over to bondage. That’s why David asked, “let them not have dominion over me!” He was concerned that he would be overtaken. Being under the dominion of sin increases our responsibility, ironically.
As believers, the way to deal with this is to submit to the law, the testimony, the precepts, the commandment, the fear, the rules of the Lord. When we submit to His Word we also learn to submit to His righteousness altogether, and we demonstrate that submission by eating His body and drinking His blood by faith. We war against taking unrighteous liberties when we receive the liberties of righteousness in Christ.