Okay, so the "Seven Deadly Sins" doesn't appear in the bible, certainly not listed as such. There is a Proverb that has something like them:
Proverbs 6:16-19 16. There are six things the Lord hates, seven that are detestable to him: 17. haughty eyes, a lying tongue, hands that shed innocent blood, 18. a heart that devises wicked schemes, feet that are quick to rush into evil, 19. a false witness who pours out lies and a man who stirs up dissension among brothers.
AND then there's
Galatians 5:19-21 19. The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; 20. idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions 21. and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God.
The list in its current form dates back to A.D.590 when Pope Gregory I revised older lists that dated back to a fourth-century monk, Evagrius Ponticus. Evagrius listed eight evil thoughts: gluttony, fornication (prostitution), avarice, pride, sadness, wrath, boasting, acedia (sloth or dejection.) Pope Gregory's version has become a kind sin standard among Catholics and many protestant denominations, including Methodists, Anglican, and Lutheran. Baptist Billy Graham has written about them. The Seven Deadly Sins are often used as fodder in movies, television and literature, but they should not be taken lightly. The Philosophy behind them dates back almost to the time of Jesus.
I had originally wanted to do a study on the Seven Deadly Sins because thinking up stuff to discuss in Sunday school is hard and I don't do hard. It's the deadly part of the seven sins that caused me to choke. Last week I called them "unforgivable" sins, but I had misspoke. The sins are certainly forgivable, Jesus can forgive any sin. The Seven Deadly Sins are Mortal sins, sometimes called Cardinal sins or Capital sins. So for the study to have any meaning, I had to define what a Mortal Sin was, and in doing so, I started down the Catholic vs Protestant rabbit hole. I had a similar experience looking up Christmas traditions and the life-long virginity of Mary.
As a Protestant (Baptist refugee,) reading Protestant commentaries and books, with a Protestant mind-set, it becomes all too easy to forget that an immense number of people read the same bible I read and come away with a vastly different interpretation. I can say with certainty, the Catholic Catechisms and laws are thoroughly considered.
Like Protestants, Catholics believe that humans are born separated from God and can be reconciled only by faith in Jesus' sacrifice on the cross. Many Protestants take a "once saved, always saved" approach, often quoting:
John 10:28-30 28. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. 29. My Father, who has given them to me, is greater than all ; no one can snatch them out of my Father's hand. 30. I and the Father are one."
Catholics, on the other hand, believe you can put yourself out of Jesus' hands through your own sinful actions. They have designated some sins as,
Venial: A lesser sin that does not result in separation from God. If left unrepented, venial sins incur time in purgatory.
Mortal: A grave sin that will destroy your relationship with God and lead to damnation if left unrepented.
There is a list of Mortal sins on the wikipedia site - https://en.wikipedia.org/wiki/Mortal_sin . I was going to include it here, but it was far too loud. And depressing. In addition to the obvious sins listed in the Galatians quote above, there are some surprising inclusions: Encouraging another to sin, Participation in Freemasonry, Heresy, Schism (failing to submit to the pope), Simony (buying/selling spiritual things like pardons and indulgences), Failing to attend Mass, Unjust Prices...
FYI, According to a 2009 study by Fr. Roberto Busa, a Jesuit scholar, the most common deadly sin confessed by men is lust, and by women, pride. It was unclear whether this is based on actual transgressions or the confessor's perception of what "counts" as a sin.