Mortal sins are defined as being grave, committed with full knowledge of its severity, and done deliberately. The second two conditions are pretty straight forward, but how does one determine the gravity of a sin?
It appears you’ve read paragraph 1857 in the Catechism. The next paragraph gives a few clues!
1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
1858** Grave matter is specified by the Ten Commandments**, corresponding to the answer of Jesus to the rich young man: “Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother.” The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
From there I would suggest reading through the Catechism section that discusses each of the 10 Commandments. It starts with #2083 and goes thru #2550.
Also, here are some Scripture passages to ponder:
1 Cor 6:9-10 Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither the immoral, nor idolators, nor adulterers, nor sexual perverts, nor thieves, nor the greedy, nor drunkards, nor revilers, nor robbers will inherit the kingdom of God.
Gal 5:19-21 Now the works of the flesh are plain; immorality, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God.
Eph 5:5 Be sure of this, that no immoral or impure man, or one who is covetous (that is an idolator) has any inheritance in the kingdom of Christ and of God.
Rev/ 22:15 Outside are the dogs and sorcerers and fornicators and murderers and idolaters, and every one who loves and practices falsehood.
Go- abhors all sin. Because he is holy and perfect, none can be in his precense. In his view, stealing an ink pen is just as grave as murder. To commit one sin is to commit them all. Praise him for his love and grace through Christ!
That’s interesting. Usually, the difficulty in determining mortal sin is the other way around.
For those under the law, breaking one law is like breaking them all: the penalty is death. But for those under grace, and not under the law, some sins are more severe than others.
Apart from this being wrong (I don’t intent to enter a debate on this, it has been covered adequately by others and clearly evident in scritpure eg Matthew 5:22), I find this to be one of the more absurd areas in which (some) Protestants differ from the Catholic faith.
Stealing a pen is as bad as murder? It’s just plain silly!