Numbers 15:22-31 ESV
 "But if you sin unintentionally, and do not observe all these commandments that the LORD has spoken to Moses,  all that the LORD has commanded you by Moses, from the day that the LORD gave commandment, and onward throughout your generations,  then if it was done unintentionally without the knowledge of the congregation, all the congregation shall offer one bull from the herd for a burnt offering, a pleasing aroma to the LORD, with its grain offering and its drink offering, according to the rule, and one male goat for a sin offering.  And the priest shall make atonement for all the congregation of the people of Israel, and they shall be forgiven, because it was a mistake, and they have brought their offering, a food offering to the LORD, and their sin offering before the LORD for their mistake.  And all the congregation of the people of Israel shall be forgiven, and the stranger who sojourns among them, because the whole population was involved in the mistake.  "If one person sins unintentionally, he shall offer a female goat a year old for a sin offering.  And the priest shall make atonement before the LORD for the person who makes a mistake, when he sins unintentionally, to make atonement for him, and he shall be forgiven.  You shall have one law for him who does anything unintentionally, for him who is native among the people of Israel and for the stranger who sojourns among them.  But the person who does anything with a high hand, whether he is native or a sojourner, reviles the LORD, and that person shall be cut off from among his people.  Because he has despised the word of the LORD and has broken his commandment, that person shall be utterly cut off; his iniquity shall be on him."
In this passage we see that there is sin with small ‘s’ (“sin unintentionally”) and then there is Sin with capital S (“with a high hand”).
The sin “with the high hand”, in today’s terms, would be “apostasy”: that is, someone who once claimed to be a believer of Jesus rejecting the faith once and for all, turning his or her back on Christ, abandoning Christ to go back to be ‘of the world’.
I’m thinking of Judas. And I’m thinking of Demas:
2 Timothy 4:10 ESV
 For Demas, in love with this present world, has deserted me and gone to Thessalonica.
I’m thinking of those false prophets that Peter speaks of:
2 Peter 2:1,20-22 ESV
 But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction ...  For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first ...  What the true proverb says has happened to them: "The dog returns to its own vomit, and the sow, after washing herself, returns to wallow in the mire."
These people who commit “apostasy”, God tells Moses, “shall be utterly cut off; his iniquity shall be on him”. There is no atonement or forgiveness for people who commit “apostasy”, because they are rejecting (once and for all) the only way in which they can be forgiven of their sins.
John also seems to make this distinction between “sin” (will be forgiven, if we ask) and “Sin” (cannot be forgiven):
1 John 5:16-17 ESV
 If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life — to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that.  All wrongdoing is sin, but there is sin that does not lead to death.
The atonement of Jesus can forgive ALL sin and wickedness of every kind — EXCEPT for the sin of rejecting that atonement. Actually, this is a glorious truth. Let us praise God for the abundant riches of His mercy:
Mark 3:28 ESV
 "Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter,
- Thank God that a person who has truly be born again cannot ultimately commit apostasy, because God protects him/her:
1 John 5:18 ESV
 We know that everyone who has been born of God does not keep on sinning, but he who was born of God protects him, and the evil one does not touch him.
- Thank God that, besides the sin of rejecting Christ’s atonement, all other sins can and will be forgiven, if we confess them to Christ. Ask God to not take His forgiveness for granted, that we may remember at what high cost we can be forgiven.
- Spend some time confessing your sins to God through Christ. Go through the Ten Commandments and ask God to forgive you for breaking all those commandments (we have all broken them at least spiritually)