According to Jesus’ own words, denying him before men will be forgiven (Luke 12:10). The believer who denies his Savior must endure Jesus’ judgment against unbelievers, something from which the faithful will be saved (Revelation 3:10), but he is forgiven. The modern critic might deny Jesus ever lived, for example, and this might be considered similar to the Jewish authorities denying Jesus’ birth was miraculous (cf. John 8:41), but such things are forgivable in the sense that someone says: “I’m sorry” and the reply is “I forgive you!” and that’s the end of it (Luke 12:10). An example in the text would be found at Luke 23:42-43 where the “good thief” had been making derogatory statements against Jesus, but afterward recanted in that he rebuked his fellow robber, and then asked Jesus to remember him when he came into his Kingdom.
The difference between speaking against Jesus, concluding he is not whom he claims to be, and saying the Holy Spirit is not what he shows himself to be is similar to the difference between a misdemeanor and a capital crime. One can be wrong about Jesus, because he believed a report written or spoken by an assumed authority. On might even have a wrong assumption about Jesus after reading the New Testament, especially while already embracing bigoted ideas about Jesus and the Bible. However, it is difficult to conceive a circumstance when the power of God is displayed for all to see—for a good purpose—that a witness could honestly claim afterward that power was really an evil power. On the one hand, we have a mistake, perhaps even a bigoted mistake, but on the other hand we have a slanderous remark made in opposition to an obvious truth (cf. Matthew 28:11-15).
One might speak against Jesus, because he had been taught that the New Testament bears a false testimony. He might speak against Jesus, because trusted ‘experts’ conclude Jesus never existed. These may be viewed as uninvestigated (honest?) mistakes—as honest as one can be without looking into the matter for oneself. However, a more selfish reason for speaking against Jesus might be due to the fact that the critic wants to behave just as he pleases, and to honestly consider Jesus would mean he would have to reconsider some of the things he says and does. In other words, I want to do my own thing, don’t bother me with the “Jesus” thing. So, while some things are said and done under more ignorant circumstances, others are said and done no matter what he might think about Jesus. Nevertheless, Jesus said all these things are simply ‘forgiven’ (Luke 12:10).
On the other hand, when someone claims that good is evil and evil is good (Isaiah 5:20), which is just another name for rebellion, because this one knows what he is doing. Those who follow him are not necessarily as guilty as he, but the prevaricator of such things knows he is in rebellion against God. It is one thing to be ignorant, but it is quite another to know what is true, but teach ignorance (cf. Luke 11:15-16).
Unforgivable does not mean one can never be pardoned. It means that what is done cannot go unpunished. A simple “I’m sorry—You’re forgiven!” won’t do. We do the same under our judicial system. If there is reason to believe a crime was committed in ignorance or through force by another party, a judge may simply pardon the offender or the district attorney may not even charge the offending party. There are many offenses that can be forgiven through apology and a positive response. However, Jesus died for the unforgivable sins, because only death could pay the price of the offending party. Jesus died in the place of the offending party for crimes that could not be forgiven after a simple “I’m sorry!”
Rebellion is an unforgivable sin. If Abraham Lincoln simply forgave the South for seceding from the Union, we’d be two nations now instead of one United States, and both might have friendlier relations with Canada than with one another. The Civil War cost many lives. It wasn’t simply forgiven. Lincoln demanded that consequences for rebellion be paid, and it was paid in the lives of American men and women on both sides of the conflict. The difference between this and Jesus is that he paid the price of our rebellion against God (an unforgivable sin) by himself. He took the whole responsibility of man’s relationship with God (1Timothy 2:6; cf. Genesis 15:12-18) upon himself and made peace through his crucifixion. That is, the unbelievers lives and the rebels lives are saved, but they are rewarded according to their works—God is not mocked (Galatians 6:7).
 In John 8:41 “WE be not born of fornication…” might be a cloaked charged that Jesus was born of fornication, a charge they make plain in John 8:48. Later Rabbinic sources make the claim that Jesus was the illegitimate son of a Roman soldier. Prior to 70 AD the greatest insult would have been Jesus had a hated Samaritan as his father, but after the Romans destroyed Jerusalem and the Jewish Temple, the greatest insult would have been that he was born of a Roman soldier.