Not all sins are equally serious. A mortal sin is a grievous offense against the law of God. A venial sin is a less serious offense against the law of God, which does not deprive the soul of sanctifying grace, and which can be pardoned even without sacramental confession.
Common sense tells us that there are different degrees of sin, for example, lying to protect a person’s feelings is on a different scale that deliberate, willful murder.
Catholic theology always starts with reason, and commonsense. Catholics observe and recognize differences among sins and call serious sins “mortal” and trifling sins “venial”. This vocabulary developed through the centuries. Scripture, of course, approves this distinction and makes use of it, clearly teaching that some sins are worse than others.
The Church distinguishes venial sins from mortal sins on a scriptural basis.
Jer. 7:26 – yet they did not listen to me, or incline their ear, but stiffened their neck. They did worse than their fathers.
Lam. 4:6 – For the chastisement of the daughter of my people has been greater than the punishment of Sodom, which was overthrown in a moment, no hand being laid on it.
Ezek. 16:44-58 – Behold, every one who uses proverbs will use this proverb about you, ‘Like mother, like daughter.’ 45 You are the daughter of your mother, who loathed her husband and her children; and you are the sister of your sisters, who loathed their husbands and their children. Your mother was a Hittite and your father an Amorite. 46 And your elder sister is Samaria, who lived with her daughters to the north of you; and your younger sister, who lived to the south of you, is Sodom with her daughters. 47 Yet you were not content to walk in their ways, or do according to their abominations; within a very little time you were more corrupt than they in all your ways. 48 As I live, says the Lord GOD, your sister Sodom and her daughters have not done as you and your daughters have done. 49 Behold, this was the guilt of your sister Sodom: she and her daughters had pride, surfeit of food, and prosperous ease, but did not aid the poor and needy. 50 They were haughty, and did abominable things before me; therefore I removed them, when I saw it. 51 Samaria has not committed half your sins; you have committed more abominations than they, and have made your sisters appear righteous by all the abominations which you have committed. 52 Bear your disgrace, you also, for you have made judgment favorable to your sisters; because of your sins in which you acted more abominably than they, they are more in the right than you. So be ashamed, you also, and bear your disgrace, for you have made your sisters appear righteous. -53 “I will restore their fortunes, both the fortunes of Sodom and her daughters, and the fortunes of Samaria and her daughters, and I will restore your own fortunes in the midst of them, 54 that you may bear your disgrace and be ashamed of all that you have done, becoming a consolation to them. 55 As for your sisters, Sodom and her daughters shall return to their former estate, and Samaria and her daughters shall return to their former estate; and you and your daughters shall return to your former estate. 56 Was not your sister Sodom a byword in your mouth in the day of your pride, 57 before your wickedness was uncovered? Now you have become like her an object of reproach for the daughters of Edom and all her neighbors, and for the daughters of the Philistines, those round about who despise you. 58 You bear the penalty of your lewdness and your abominations, says the LORD.
1 Jn. 5:16 – if see brother committing what is not mortal sin.
Mat. 11:22 – But I tell you, it shall be more tolerable on the day of judgment for Tyre and Sidon than for you.
Jn. 19:11 – Jesus answered him, “You would have no power over me unless it had been given you from above; therefore he who delivered me to you has the greater sin”.
Some sins are so great that they exclude the sinner from God’s Kingdom.
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 idolaters, nor adulterers, nor sexual perverts, 10 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: fornication, impurity, licentiousness, 20 idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, 21 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.
Others are less serious, deserving temporal punishment, but not eternal fire.
1 Cor. 3:11, 15 – For no other foundation can any one lay than that which is laid, which is Jesus Christ. … 15 If any man’s work is burned up, he will suffer loss, though he himself will be saved, but only as through fire.
Small sins and faults are found in even holy people.
Eccl. 7:21-22 – Do not give heed to all the things that men say, lest you hear your servant cursing you; 22 your heart knows that many times you have yourself cursed others.
Jas. 3:2 – For we all make many mistakes, and if any one makes no mistakes in what he says he is a perfect man, able to bridle the whole body also.
1 Jn. 1:8 – If we say we have no sin, we deceive ourselves, and the truth is not in us.
Before the 11th century, the degrees of gravity among sins were recognized with ever growing clarity.
Origen [post AD 244] “In regard to grave crimes a place for repentance is conceded only once. Those, however, which are common, and into which we frequently fall, always admit repentance, and are forgiven without cease” (Homilies on Leviticus 15, 2).
Council of Elvira [AD 300] “It is determined that if someone, by a grave lapse, falls into the ruin of death, he may not do penance in the presence of a presbyter, but rather in the presence of a bishop. However, if constrained by illness, it is necessary, a presbyter may give him communion, – or even a deacon, if the priest command him” (Canon 32).