What about sinning against the spirit talked about in the book of Hebrews?
Hebrews 6:4-8: "It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace. Land that drinks in the rain often falling on it and that produces a crop useful to those for whom it is farmed receives the blessing of God. But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned. Even though we speak like this, dear friends, we are convinced of better things in your case — the things that have to do with salvation."
Hebrews 10:26-29: "For we—sinning wilfully without compulsion after receiving the full knowledge of the truth, there remaineth no more sacrifice for sins, but only a fearful expectation of judgment and fiery zeal about to consume the enemies of God. Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. How much more severely do you think those deserve to be punished who have trampled the Son of God underfoot, who has considered as an unholy thing the blood of the covenant that sanctified them, and who have insulted the Spirit of grace?"
Does this mean that someone who has come to know God and has drifted away from His will cannot be forgiven?
I took this information from my bible commentary.
Hebrews 6:4-8
These people had once been "enlightened". It is never clearly stated that they had been born again. They had heard the gospel,so they were not in darkness concerning the way of salvation. They tasted the heavenly gift: Jesus. But they had never received Him by a definite act of faith. It is possible to taste without eating or drinking. It is not enough to taste Christ; unless we eat the flesh of the Son and drink His blood: receive Him as Lord and Savior.
Partaking of the Holy Spirit: remember that the Holy Spirit carries on a pre-conversion ministry in men. Conviction and judgment, then leading them to repentance and points men the Christ. Men may partake of the Holy Spirit's benefits without being indwelt by Him.
They had tasted the good word: they had heard the gospel. Were moved by it and drawn to it. But they were like the seed that fell on the rocky ground. They received the word, but it did not take root.
Powers of the age to come: powers = miracles. Age to come = Millennial age. The miracles they witnessed were a foretaste of signs and wonders which will be performed in Christ's kingdom.
If they fall away; put Him to open shame: signifies a deliberate, malicious spurning of Christ, not just a careless disregard for Him. Indicates a positive betrayal of Him. These people are apostates. They hear the gospel, profess being a Christian, then abandon their faith and take their place with enemies of the Lord. It is a sin that can be committed only by unbelievers. By those who knowingly, willingly and maliciously turn against the Lord.
The true believer is like the land which drinks in the rain. Bringing forth useful vegetation and is blessed by God. The apostate is like land that also is well watered but bears nothing but thorns and briers, the fruit of sin. It receives but never produces. Such land is worthless. It's destiny is to be burned.
Hebrews 10:26-29
After receiving the knowledge of truth, it is deliberately sinning. Apostasy. For such a person, there no longer remains a sacrifice for sins. That person has decisively and conclusively rejected the once-for-all sacrifice of Christ. Therefore God has no other way of salvation to offer him. All sin is willful, but here it speaks of apostasy, as a willful sin of extraordinary seriousness.
Certain fearful expectation and judgment: there is no hope of escape. It is impossible to renew the apostate to repentance. He has knowingly and willingly cut himself off from God's grace in Christ. His fate is fiery indignation which will devour the adversaries. Denoting punishment that is dreadfully severe. God classes apostates with adversaries. This indicates positive opposition to Christ, not a mild neutrality.
The enormity of his sin is seen in the three charges that are leveled against him:
1 - he has trampled the Son of God underfoot. After professing to be a follower, he now brazenly asserts that he wants nothing more to do with Him. Denying any need of Christ as Savior, positively rejects Him as Lord.
2 - He has counted the blood of the covenant by which he was sanctified a common thing. He counts as useless and unholy the blood of Christ. He had been set apart by this blood through his association with Christian people, he had been sanctified just as an unbelieving husband is sanctified by his believing wife (1 Corinth. 7:14) but that does not mean he was saved.
3 - He has insulted the Spirit of grace. The Spirit of God had illuminated him concerning the good news; convicted him of sin; and pointed him to Christ. But he had insulted the gracious Spirit by utterly despising Him and the salvation he offered.
I hope this make some sort of sense to you. I think to sum it up: these passages speak of someone who pretended to accept Christ, attended church but didn't really mean it in his heart. He wasn't saved to begin with. And after hearing the good news, having the knowledge of salvation, denies it; denies Christ and then makes a willful effort to discredit Him.