"Truly I tell you, people can be forgiven all their sins and every slander they utter, but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.” He said this because they were saying, “He has an impure spirit.” Mark 3:28-30
The parallel to these verses is Matthew 12:30-32:
"Whoever is not with me is against me, and whoever does not gather with me scatters. Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
In these verses Jesus Christ identifies two types of sin, venial and mortal, or what St. John defines as deadly:
"If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing as deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly." (1 John 5:16-17).
While venial sin is always offensive to God, thus weakens grace in the soul and damages our relationship with God, it is not deadly, meaning it is still forgivable. However, a person who frequently indulges in venial sin is very likely to fall into mortal sin if they persist in their evil ways.
Mortal sin is always a grave matter. St. Paul states that anyone who commits mortal sins willingly and with full consent shall not enter the kingdom of God. In Galatians Paul provides a list of mortal sins:
"Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God" (Galatians 5:19-20).
Paul also tells the Corinthians,
"....know you not that the unjust shall not possess the kingdom of God? Do not err: neither fornicators, nor idolaters, nor adulterers, Nor the effeminate, nor liers with mankind, nor thieves, nor covetous, nor drunkards nor railers, nor extortioners shall possess the kingdom of God" (1 Corinthians 6:9-10).
Based on Jesus Christ’s interpretation of the gravity of the Ten Commandments, grave sins fall under the heading of preference for self (idolatry) rather than love of God (Mark 12:30), hatred of immutable truth and unchanging moral law, apostasy, envy, covetousness, blasphemy, malice, heresy, voluntary murder (Genesis 4:10 ), presumption of self-creation/self-salvation (evolutionary thinking, the Magic Way, etc.) and sin of impurity against nature –sodomy and homosexual relations (Genesis 18:20):
"...the homosexual movement has a visceral hatred of Judeo-Christian religious values. We see it in their frequent angry and venomous outbursts against churches and religious people (because) traditional religion and the Bible have been society's spiritual barrier against the whole range of "gay" sexual practices and the modern LGBT political movement. Religion instructs us as to why we cannot accept their behaviors as normal or natural (meaning, of nature -- a creation of God). (Boston Gay Pride - Part 2: Attacking traditional religion and the Catholic Church, http://www.massresistance.org/docs/gen2/13b/pride_week/parade-festival/religion/index.html )
The living, personal God and the Bible, as understood by traditional Christians throughout the whole body of the Church together with faithful Jews, is anathema to the homosexual movement. They can't hold their feelings back and toward the Catholic Church they willfully hold a particularly malevolent hatred as evidenced by the 'gay' servants of darkness, the blasphemous Sisters of Perpetual Indulgence,
"(a) group of men who dress as hideous caricatures of nuns and come to homosexual events to insult the Catholic Church as much as possible, to the cheering and applause of the homosexual community and its allies." (see photos at MassResistance link)
Mortal sin is extremely grave because willful. It is not due to ignorance but to willful hatred of immutable truth and unchanging moral law. Mortal sin is willfully committed by one who knows their sin is wrong, but who with deliberation commits the sin anyway as with the Sisters of Perpetual Indulgence and with Karl Marx who willed himself to not know of God's existence despite knowing that his life is by God's grace. (Political Apocalypse: A Study of Dostoevsky's Grand Inquisitor, Ellis Sandoz, pp. 113-114)
In his youth Karl Marx professed to be a Christian. His first written work is called "The Union of the Faithful with Christ." Speaking of his love for Christ and how this love turns the heart of man toward charity for one's brethren, Marx wrote:
"Through love of Christ we turn our hearts at the same time toward our brethren who are inwardly bound to us and for whom He gave Himself in sacrifice." (Marx and Satan, Richard Wurmbrand, p. 11)
Though Marx began life with a profession of love for Jesus Christ, not long after graduating from high school a malevolent side began to emerge until in his drama "Oulanem" Marx does what the Devil does: he consigns the entire human race to damnation. Wurmbrand comments:
"Oulanem is probably the only drama in the world in which all the characters are aware of their own corruption, and flaunt it and celebrate it with conviction. In this drama there is no black and white...Here all are servants of darkness....all are Satanic, corrupt, doomed." (ibid, p.19)
In his letter to the Hebrews St. Paul warns us that even though with God all things are possible (Mark 10:27), as a practical matter of evangelism it is impossible for men to renew again to repentance mortal sinners like Marx because though once enlightened and Marx had tasted of the heavenly gift and been made a partaker of the Holy Spirit, Marx himself declared the hardness of his heart with his willful rejection of God whereby he has publicly denounced Jesus, thereby declaring that He deserved crucifixion and humiliation. Thus if like Marx,
"...we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will 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 someone deserves to be punished who has trampled the Son of God underfoot, who has treated as an unholy thing the blood of the covenant that sanctified them, and who has insulted the Spirit of grace? For we know him who said "It is mine to avenge: I will repay," and again, "The Lord will judge his people." "It is a dreadful thing to fall into the hands of the living God." Hebrews 10:26-31
Despite all that's been said thus far, grave sins are not the unforgivable sin. They only become unforgivable if in the hardness of a sinners heart he dies in his grave sin, meaning that the unforgivable sin is impenitence (dying in a state of stiff-necked obstinancy), a teaching that goes back to the time of Augustine. In the Summa, Thomas Aquinas delineates Augustine’s teachings dealing with unforgivable sin:
“Augustine says . . . (Enchiridion lxxxiii) that ‘he who dies in a state of obstinacy is guilty of the sin against the Holy Ghost,’ and (De Verb. Dom., Serm. lxxi) that ‘impenitence is a sin against the Holy Ghost,’ and (De Serm. Dom. in Monte xxii), that ‘to resist fraternal goodness with the brands of envy is to sin against the Holy Ghost,’ and in his book De unico Baptismo (De Bap. contra Donat. vi, 35) he says that ‘a man who spurns the truth, is either envious of his brethren to whom the truth is revealed, or ungrateful to God, by Whose inspiration the Church is taught,’ and therefore, seemingly, sins against the Holy Ghost.” (ST 2b:14:2, Sed Contra).
While it is impossible for men to renew a hardened heart, until death it is still possible for the Sisters of Perpetual Indulgence to repent of their sins, for a heretic to renounce his heresy, a liar his hatred of truth, and an apostate to re-embrace the faith of Christ because with God all things are possible:
“With men it is impossible, but not with God; for all things are possible with God” (Mark. 10:27).
The living God ultimately honors our choices, whether for good or evil. Thus if the hardened sinner refuses to repent, then his envy, covetousness and self-idolatry "blasphemes against the Holy Spirit (and) will never be forgiven; they are guilty of an eternal sin.”
By their own choice, the impenitent willfully shut themselves out of paradise, thus have no one to blame but themselves. C.S. Lewis makes this point, saying that Hell is of the willful choice of the unrepentant sinner:
"There are only two kinds of people in the end: those who say to God, 'Thy will be done,' and those to whom God says, in the end, 'Thy will be done.' All that are in Hell chose it."