According to the website, the 6 Sins against the Holy Ghost are...
Presumption
Despair
Resisting the known truth
Envy of another’s spiritual good
Obstinacy in sin
Final impenitence
Not all of these 6 sins against the Holy Ghost are the "unforgivable sin." Only the last one, final impenitence, is the sin that will keep you from heaven. From what I've read in scripture, however, the "unforgivable sin" is
blasphemy against the Holy Spirit - that is, knowingly attributing the power of God to the power of Satan, as the Pharisees did when they accused Jesus of dispelling evil spirits by the power of Beelzebub.
Mark 3:28-30: "Truly I tell you, people will be forgiven all their sins and all the blasphemies they utter. But whoever blasphemes against the Holy Spirit will never be forgiven, but is guilty of an eternal sin. He said this because they [the Pharisees] were saying, ‘He has an evil spirit’."
Matthew 12:30-32: "And so I tell you, people will be forgiven every sin and blasphemy. But the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
Luke 12:8-10: "I tell you, whoever acknowledges me before men, the Son of Man will also acknowledge him before the angels of God. But he who disowns me before men will be disowned before the angels of God. And everyone who speaks a word against the Son of Man will be forgiven, but anyone who blasphemes against the Holy Spirit will not be forgiven."
In the context of all three of these passages the Pharisees ascribe Our Lord's miraculous works to Satan. While the Church teaches that there are no limits to God's mercy, anyone who engages in this kind of pride rejects the Holy Spirit, which can lead to hardness of heart, which can lead to final impenitence, which results in eternal loss.
- Lisa