Mortal and Venial Sin
Catholic Church teaches that there are two categories of sin .*
The first is mortal sin [1855, 1874]:
As original sin killed the life of God in Adam, so mortal sin ends the life of God in baptized Catholics . That is why the Church calls this kind of sin mortal, from the Latin word for death. Mortal sin kills the soul and, unrepented, brings eternal punishment [1033, 1861, 1874].
The second kind of sin, according to the Church, is venial sin [1862, 1875]:
The Church calls these sins venial, from the Latin word venia, meaning pardon. God will forgive the sinner of these minor sins if he confesses them to God in prayer with sincere repentance. Venial sins weaken a person’s spiritual vitality and make the individual more susceptible to greater sins. But unlike mortal sins, they do not kill the life of the soul or incur eternal punishment. 
Roman Catholic theologians compare the manner in which mortal and venial sins affect the soul to the way in which illnesses affect the body. Most ailments are minor. The body’s immune system fights them off and eventually restores health. A venial sin is like a minor sickness of the soul. It hinders spirituality and lowers resistance to temptation, but the vitality of the soul survives. 
Mortal sin is a death blow. It kills the soul as surely as a fatal disease kills the body. When a Catholic who has received sanctifying grace through baptism commits a mortal sin, he loses that grace. 3 He who by baptism had been justified, because of mortal sin forfeits the grace of justification, or, it might be said, is dejustified.4 He who had been a child of God and on his way to heaven, becomes a child of wrath and destined for hell. [1033, 1861, 1874] And just as a dead body has no capacity to restore life to itself, the Church teaches that a soul struck dead by mortal sin cannot revive itself. The sinner must turn to the Church and to the sacrament of penance. [1446, 1856] 5
All Sin Is Mortal
Just how serious is sin? The Bible teaches that the spiritual consequence of every sin is the death penalty, eternal separation from God in the Lake of Fire (Revelation 20:14-15); "The soul who sins will die," (Ezekiel 18:4); "The wages of sin is death" (Romans 6:23).
This is not to say that every sin is equally wicked or abhorrent to God. Scripture teaches that some sins are more evil than others and will be judged accordingly (John 19:11; Matthew 10:15). Jesus taught that there will be degrees of eternal punishment in hell (Luke 12:47-48).
Nonetheless, the Lord never distinguished between sins in terms of their ultimate penalty. 6 Jesus taught that every sin warrants eternal punishment in hell. He taught that the sin of anger brings the same punishment as the sin of murder (Matthew 5:21-22), and the sin of lust the same penalty as the sin of adultery (Matthew 5:27-30).
Roman Catholicism, on the other hand, teaches that some sins are "light sins," minor infractions of the moral laws of God [1862-1863]. Telling a small lie or stealing something inexpensive is somehow different from telling a big lie or committing grand theft. Small sins, venial sins, do not bring eternal punishment.
Additionally, the Roman Catholic Church teaches that, though venial sins may incline a person toward later committing a mortal sin, not even the regular practice of venial sins warrants eternal punishment. A baptized Catholic who does not commit a mortal sin remains in a state of grace even if he is habitually guilty of a multitude of venial sins. 
The Scriptures, on the other hand, teach that if a person’s life is characterized by sin, he should not consider himself a born-again Christian:
The Church even says that if mitigating circumstances exist, not even the gravest sin merits eternal punishment. It teaches that for a sin to be mortal, three conditions must be met: [1857-1862]
Should the sin not meet one of these requirements, it does not merit eternal punishment no matter how evil the act might be.
In practice these conditions become ready-made excuses for lawlessness. Consider, for example, a person who has displayed a low ability to resist a certain sin. According to Roman Catholic theology, his sin may not be completely deliberate: "The freedom of our will can be impeded by our natural disposition, the influence of improper upbringing, internal or external compulsion, or the force of violent and sudden passion."7 If a person in such a condition were to commit a gravely evil sin, therefore, it is not a mortal sin. Father Melvin L. Farrell shows how this might apply to sexual temptation:
Consequently, though the Bible teaches that all sins are mortal, the Church teaches that no sin is necessarily mortal. And, though the Bible never mentions venial sin, the Church teaches that every sin is potentially venial!
An Underestimated Problem
In the Sermon on the Mount, Jesus taught, "If your right eye makes you stumble, tear it out, and throw it from you.... If your right hand makes you stumble, cut it off, and throw it from you..." (Matthew 5:29-30). Why such severe treatment? "For it is better for you that one of the parts of your body perish, than for your whole body to be thrown into hell" (Matthew 5:29). The eternal consequences of sin are so horrifying, that, if physical mutilation could keep one from sinning, it would be a better alternative than ending up in hell.
God allows the full weight of sin to fall upon the sinner that he might cry out, "What must I do to be saved?" (Acts 16:30). It is the convicted sinner who realizes that he needs a Savior. This is the person who upon learning the gospel clings to the cross like a drowning man to a life preserver. This is the person who knows that though he deserves to go to hell a thousand times over, Jesus’ blood has made full satisfaction for his every sin: past, present, and future.
Roman Catholic theology, on the other hand, consistently undermines the seriousness of sin and its consequences, most notably by teaching that most sins are not punishable by death.
This is a lie that is as old as the world itself. Soon after God warned Adam and Eve that if they disobeyed him they would "surely die" (Genesis 2:17), Satan told Eve, "You surely shall not die!" (Genesis 3:4).
This same lie has deceived countless Roman Catholics. As a result, most go through life unaware of the magnitude of their guilt before God. Underestimating their problem, they readily embrace an inadequate and faulty solution: the gospel according to Rome.
Adapted from The Gospel According to Rome by James G. McCarthy, Harvest House Publishers, © 1995.
The process by which baptized Catholics "through sin have forfeited the received grace of justification" is summarized here as dejustification. The process by which Catholics "can again be justified" is expressed here as rejustification.
Dejustiflcation should not be understood as a return to that state in which the person existed before baptism. The Church teaches that baptism imprints an indelible character upon the soul. [1272-1274, 1280] Additionally, the sacrament is conferred principally to remove original sin. When a person commits a mortal sin, though he loses justifying grace in his soul, original sin does not return and the imprint of baptism is not removed. Therefore, a person can be baptized only once and grace forfeited after baptism must be restored through the sacrament of penance.
Similarly, rejustification should not be understood to mean the removal of original sin, the imprinting of the character of baptism, and other effects unique to baptism. Rather, it is used here to refer to the restoration to the life of grace through the sacrament of penance. (Cf. Council of Trent, session 7, "Canons on the Sacraments in General," canon 9; Thomas Aquinas, Summa Theologica, part III, question 66, article 9.)
Perfect contrition, explains the Church, is one such way: "When it arises from a love by which God is loved above all else, contrition is called ‘perfect’ (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible."  
This interpretation ignores the context of the passage. The epistle is written to Christians influenced by the heresy of Gnosticism. False prophets were teaching that only the spiritual realm mattered. One’s behavior in the flesh was irrelevant. They even denied that Jesus had come in the flesh.
John exhorts the Christians to hold fast to the truth. He assures them that they can pray with confidence that God will hear and answer their requests (1 John 5:14-15). John, however, does make one exception to this promise: "If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this" (1 John 5:16). The Christians were not to intercede in prayer for those who had renounced Christ and embraced Gnosticism. The reason for this restriction is that there is no forgiveness for those who reject God’s only provision for sin, Jesus Christ the Savior (Hebrews 6:4-8).
1 John 5:17, therefore, is not speaking about different punishments for sin, but rather, a special condition when intercessory prayer is inappropriate. If a person commits a "sin leading to death" (1 John 5:16), here the sin of apostasy, no intercession is to be made, for God is not willing to grant that request.
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