There is definitely a difference between willful sin (rebellion) and mistakes. All sin isn't rebellion. Christians can do both, and they can also repent of both.
What David did was rebellion; he willfully sinned against the LORD ("utterly scorned" Him, 2 Samuel 12:14), knowing full well what he was doing. And he had plenty of time to think about it while he was conspiring to murder Bathsheba's husband. He was able to shut out the convictions of the Holy Spirit, and the Law of the LORD, which he knew, in order to carry out this sin that he had his heart set on--a testament to the capacity of the sinful nature, even in saints; even in a saint like David.
(The sinful nature is no less depraved in a saved person than it is in an unsaved person. God knows what it is capable of better than we do. We underestimate it, but God, thankfully, does not.)
Even though David rebelled, he repented, and God forgave him, even though He chastened him severely. And Christians who rebel against God, can repent and be forgiven---if it is not a sin that leads to death.
The indictment and outrage that the Lord has against most of His saints today is that they DO know that what they are doing (or are not doing as they should be) is wrong, but they are justifying what they are doing to themselves, refusing to listen to His correction and avoiding conviction by all means, whether by hardening themselves against the voice and promptings of His Spirit, or by picking and choosing what they read in His Word to suit themselves, or by gathering to themselves---intentionally---teachers who will tell them what they want to hear, and despising or ignoring the chastening of the Lord.
The Bible refers to the time preceding the revealing of the antichrist as the REBELLION (both in Daniel and in 2 Thessalonians)---of Christians (not unsaved people)---because, just like in the time of Jeremiah and Ezekiel and the other old testament prophets, the spirit of His people is a spirit of defiance, indifference, and stubbornness.
This is not a time of Christians merely making mistakes and being ignorant; they know what the word of God says and they do not put up with it. They twist it and defy it, or just refuse to read it.
And their ignorance about many things is a result of them not applying what they DO know so that God might add to their understanding of things they need to know, as well as a result of their not seeking for understanding (because they are busy doing other things they deem more important---like having fun or making more money.)
They do know (or could know and should know) what God wants them to do and what He commands them to do---as do/could their pastors---but they do not do it. Because they love themselves, and money, and pleasure, etc., just as the Scriptures declare that Christians will in the last days, when the love of most grows cold.
The most horrible thing about this is that they have the grace of God available to them to obey Him, all day long, day after day, and they are squandering it, and forcing Jesus Christ---who died to give them that grace, and to whom they belong---to endure their sins, as His name is blasphemed by the unsaved because of the evil, irreverent and foolish behavior of apostate Christians. Do you have any idea how angry and sad that makes our Lord? (They don't. And they don't even care. WRETCHED.)
And they won't be able to blame their pastors for their rebellion, because they choose to listen to them over the word of God. The pastors will be judged for sure (remember Jesus' warning about the one who causes one of His little ones to sin being better off having a millstone hanged around his neck and drowned in the sea? And the warning in James to those who teach doctrine?); but so will those who listen to their pastors instead of God.
We all have His Word; we can read it. We all have His Spirit; we can listen to Him. So there is no excuse. It's just plain old rebellion.
An appalling and horrible thing has happened in the land:
the prophets prophesy falsely,
and the priests rule by their own authority;
AND MY PEOPLE LOVE IT SO...
(Jeremiah 5:30,31)
For they are a rebellious people, lying children,
children unwilling to hear the instruction of the LORD;
who say to the seers, "Do not see,"
and to the prophets, "Do not prophesy to us what is right;
speak to us smooth things, prophesy illusions,
leave the way, turn aside from the path,
let us hear no more about the Holy One of Israel." (Isaiah 30:9-11)
This is the spirit of the times among Christians.
What Jesus said about those who do evil not coming to the light for fear of their sins being exposed is not just true of the unsaved; Christians who are willfully sinning run from Him too. They run from His word, they run from His prophets, they try to muffle His Spirit. They don't want to be convicted because their hearts are set on themselves. They know they're doing wrong and they feel guilty, but they aren't penitent. They just want to get away from the discomfort of their conviction because they've already decided to keep doing what they're doing.
They want talk about Jesus, do things "for Him", wear Him on their clothes and jewelry (the irony), but they don't really want Jesus. They want to do what they want to do, and they don't want Jesus to interfere. All their Christian bric-a-brac, Christian activities, and empty talk is fig-leaves. God sees right through it, and will judge them accordingly.
And they wonder why I don't have anything good to say to most of them. I wish I did. But these are not good times, just as Jeremiah's times were not good times. They are spiritually PERILOUS times.
God's people don't need to hear any more sweet nothings; they have plenty of people to tell them such things. They need to hear the truth for their own good, even if they don't want it (which most of them do not). And the truth is that those who are compromising what they know to do and those who are rebelling against God need to repent, now. THAT is what the Lord is saying to the majority of His people and what they need to hear---NOT jokes, cute anecdotes, lying visions, and God's promises without His conditions.