In Catholicism, there isn't a single "greatest sin," but rather the gravest are mortal sins that separate one from God, especially sins of malice or deliberate evil, with blasphemy against the Holy Spirit (attributing His work to the devil) considered particularly unforgivable if unrepented, alongside other sins crying to Heaven for vengeance, though all sins can be forgiven through repentance and confession. Pride is often seen as the root of all sin, leading to self-deification, as exemplified by Lucifer.
They are not explicitly listed in the Bible but developed within early Christian theological tradition, drawing on various biblical passages. In the standard list, the seven deadly sins according to the Catholic Church are pride, greed, wrath, envy, lust, gluttony, and sloth.
One eternal or unforgivable sin (blasphemy against the Holy Spirit), also known as the sin unto death, is specified in several passages of the Synoptic Gospels, including Mark 3:28–29, Matthew 12:31–32, and Luke 12:10, as well as other New Testament passages including Hebrews 6:4–6, Hebrews 10:26–31, and 1 John 5:16.
The two great sins emphasized throughout the Old Testament are failings relative to the two great commandments: to love God and to love one another. The two greatest sins are failings of love. In short, then, you cannot keep the commandments if you do not love God and if you do not love others.
With this declaration, Alma identified for Corianton the three most abominable sins in the sight of God: (1) denying the Holy Ghost, (2) shedding innocent blood, and (3) committing sexual sin. Adultery was third to murder and the sin against the Holy Ghost as abominable sins.
Fornication is not the unforgivable sin, neither does it consign a person to a life of second-class spiritual citizenship. Think of the second chance given to the prostitute Gomer in the book of Hosea. Think of the sexual sinners in the genealogy of Jesus.
The seven sins are pride, greed, lust, gluttony, envy, sloth, and wrath. The theory here is that most of the sins that you're engaged with on a daily basis have their roots in one of these seven big sins. If you can focus on those seven things, your life will be much, much holier.
Pride Is the original sin. It was Satan's sin In Heaven, and the sin he promotes among many humans today. The cure for pride Is cultivating humility. Humility Is being honest about who we really are before God.
Reserved Sins?
Now, the Catechism states that failure to respect God in one's speech is blasphemy, which it says is a “grave sin” in itself (aka grave matter). Therefore, to thoughtlessly say “Oh my God” as a mere expression is a sin of grave matter.
In Mark 3:29 Jesus says that “whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.” Matthew's account adds that even blasphemy against the Son of Man will be forgiven, but not blasphemy against the Holy Spirit (Matthew 12:31–32).
A Mortal Sin
On New Year's Eve 1930, the Roman Catholic Church officially banned any "artificial" means of birth control. Condoms, diaphragms and cervical caps were defined as artificial, since they blocked the natural journey of sperm during intercourse.
Therefore gluttony is a sin. I answer that, Gluttony denotes, not any desire of eating and drinking, but an inordinate desire. Now desire is said to be inordinate through leaving the order of reason, wherein the good of moral virtue consists: and a thing is said to be a sin through being contrary to virtue.
John Cassian and St. Gregory the Great. They are called “capital” because they engender other sins, other vices. They are pride, avarice, envy, wrath, lust, gluttony, and sloth or acedia.
Lying is another sin which is classified among the Greater sins. The famous scholar, Shaykh al-Ansari (r.a.) writes in his book, Makasib al-Muharrama: “Common sense suggests lying is Haram, all the revealed religions also say so; particularly Islam.
Onan refused to perform his duty as a levirate and instead "spilled his seed on the ground whenever he went in" because "the offspring would not be his", and was thus put to death by Yahweh. This act is detailed as retribution for being "displeasing in the sight of Lord".
If you say it intending to profane the sacrament of marriage, it is mortal sin. If you say it casually/carelessly, it would be venial. For instance, if you tell someone to go F themselves and mean it, that would be nearer to mortal sin than stubbing your toe and reacting by shouting F!
There are a few sins reserved to higher-level authorities to absolve. They are generally rare: if you assault the pope or consecrate a bishop without his approval, you need absolution from the Vatican, not just your parish priest.
GREED or AVARICE: The inordinate love of and desire for earthly possessions. Amassing a fortune and desiring to accumulate more than others is greed. Next to anger, lust and envy, more crimes have been committed due to greed than any other deadly sin.
So now to answer the question, “Are sins of thought really as bad as committing the actual sin?” Yes. Jesus is clear. Praise God for that truth, because otherwise our sinful natures might convince us that we can do enough to enter heaven. The lesson of Jesus is that even our thoughts condemn us.