Sin is a mystery, and it’s a mystery the psalmist wrestles with in Psalm 19. First he looks up to the heavens to delight in the Creator’s handiwork (verses 1–6). Then he looks down to delight in God’s words (verses 7–11). And the next moment he is on his face pleading with God for power for victory over sin (verses 12–13).
12 Who can discern his errors?
Declare me innocent from hidden faults.
13 Keep back your servant also from presumptuous sins;
let them not have dominion over me!
Then I shall be blameless,
and innocent of great transgression.

Sin in Two Forms

The psalmist shows us sin in at least two different forms: "hidden faults" and "presumptuous sins." One is like a trapdoor that swings out from underfoot, and the other is like a double-door seen from a distance and approached. This post will focus on the second of these, on "presumptuous sins" (ESV, KJV, NASB), or "willful sins" (NIV, HCSB), or "deliberate sins" (NLT). But what are they?
A number of commentators believe the "great transgression" at the end of the passage refers to physical adultery or spiritual adultery (idolatry). No doubt these serious sins are included, but willful sins come in various shapes and sizes (see Exodus 21:14,Numbers 15:30–31, Deuteronomy 17:12–13). We must go deeper than merely cataloging “bad” sins.
More generally, it appears that presumptuous sins arise from carelessness with God and his word, and carelessness with the needs of others. We can also be lured into these sins by the willful disobedience of others. Whatever the origin, over time our carelessness leads to callousness, and calloused hearts lead to arrogance or insolence towards God and others.

Do You Do This?

So is a born again Christian susceptible to presumptuous sins that knowingly contradict God’s will?
First, notice here that the believing, regenerate author of this psalm believed he was not only susceptible to willful sins, he was susceptible to bondage to them.
Christians are especially prone here, writes Charles Simeon, because these are “any sins whatever that are committed against light and knowledge, or on a presumption that God will not punish them in the eternal world.” Presumptuous sin is a misuse of revelation and the gospel and they assume a religious commitment of some level.
John Calvin concurs. The believer, who at one moment groans under the burden of remaining sin and who is aware of the seriousness of sin, is still capable of falling into willful sin, a sin that contradicts what he knows to be true. Calvin senses in the psalmist’s prayer that “unless God restrain us, our hearts will violently boil with a proud and insolent contempt of God.” God help us.
There is nothing safe about living in a sin you know to be wrong.

Self-Destruction

Presumptuous sins are self-destructive and should be avoided at all costs. “Christian, when you sin presumptuously,” writes Thomas Watson, “you do what in you lie to kill the babe of grace in your soul!”
Says Pastor John,
David sees a difference between, on the one hand, sins that we commit because they baffle us and sneak up on us ["hidden faults" of verse 12], and on the other hand, sins that we commit because we presume to know better than God or presume that sin is no big deal ["presumptuous sins" of verse 13].
The point is not that there is a special category of extra-bad sins, like murder, rape, treason, etc. The point is that there is a special category of sinning — namely, sinning in arrogant defiance of a known law. It’s not so much what you do that puts sinning in this category as whether you do it with forethought and defiance and rebellion. This is what David calls presumptuous sins. They are fully intentional, with our eyes open, and with a heart that says, 'I know God says this is wrong and harmful, but I just don’t care what God thinks; I’m going to do it anyway.' (Sermon)

The How and Hope of Fighting

So how do we fight presumptuous sins?
Do not be content with whispering your sin to God. That is good. Very good. But he offers us something more: “Confess your sins to one another and pray for one another, that you may be healed” (James 5:16). There is a release and healing that flows from confessing not only to God in the secret place of your heart, but also to a trusted friend, or to the person you have offended. The tender words, “I’m sorry, will you forgive me?” are one of the surest paths to joy. (When I Don't Desire God, 224)
Pastor John holds out hope for us all.
…we must and we can get victory over presumptuous sins, even while we go on wrestling with baffling corruption. Therefore the focus here is on praying for power: “Keep back your servant from presumptuous sins; let them not have dominion over me.” I believe we can experience complete triumph over presumptuous sin, and that presumptuous sinning must cease to be the characteristic of our lives. God calls us to this. He gives us the power for it through the Holy Spirit. (Sermon)
Thus we return to Psalm 19:12–13, and the plea for strength. With God’s grace, we can make it our aim to live blameless and innocent of great transgressions.