Who can understand his errors? Cleanse them me from secret faults. (Psalms 19.12)

For those who teach that one has to repent for all his sins to be forgiven even as believers, we have riddle. How can they ask forgiveness for sins they are not aware of? For reasons only known to himself, our Lord keeps some our sins out of our awareness. God has secrets. 'The secret things belong to the Lord our God; but those things which are revealed belong to us and to our brethren forever, that we may do all the words of this law' (Deut. 29:29).

The Hebrew word translated understand means to discern. It is more than a mere acknowledgement of something, it is an insight into it. David is asking a rhetorical question; one he cannot answer!

Brethren, it is a question no man can answer! It puts coffin lid to any type of perfectionism or progressive sanctification! We see this thought reflected in the words of Job. 'I know that you can do everything and that no thought can be withheld from you. Who is. that hides counsel without knowledge? therefore I have uttered that I undertood not; things which were too wonderful for me' (42.1-3). Even though we are not strictly fools in the Biblical sense of the term (this is term reserved for unbelievers), it can still apply to us in our limited understanding of our sinful self a the GREAT work of Christ on our behalf, 'you have set our iniquities before you; our secret sins in the light of your countenance' (Psalm 90:8). What does Jeremiah mean by what he says in 17:9, 'The heart is DECEITFUL ABOVE ALL THINGS and desperately wicked; WHO CAN KNOW IT?' The Hebrew word translated desperately means sick or wounded. In this case it means incurably wounded, per (BDB, Hebrew and English Lexicon). It is the same question brethren. Who can know it? Who can understand it?

There is only one answer. We must be cleansed. Not cleansed of some individual sins as we go along in life, but all of them at once. We must be washed in the blood of the lamb! Those secret sins is a Hebrew Niphal perfect participle. It refers to either a sin which we try to hide and do in secret or a a sin that is hidden from our deceitful hearts and is unknown to us. As Christians we must confess that we are sinners but we repent once through faith. Speaking of the Piel verb translated "cleanse me" grammarians write:

lt involves some kind of proclamation, delocution, or estimate assessment, although the precise nature of these verbs and their relationship to the factitive is debated...(literally: declare me innocent) of hidden faults (Psalm 19.13) (Bill Arnold & John Choice, A Guide To Biblical Hebrew Syntax, 2nd edition, Cambridge University Press, 2018, pg. 56)

So according to the grammar David is asking for God to declare him innocent; it is a legal declaration; it is a justification. This only happens through the atonement of the Son of God where in God looks at us as righteous; he declares us righteous in Christ.

The NET Study Bible correctly states on this passage: "The rhetorical question makes the point that perfect moral discernment is impossible to achieve. Consequently it is inevitable that even those with good intentions will sin on occasion."