On what basis does the Church classify sin to greater or lesser degrees?
The Catechism states that “sins are rightly evaluated according to their gravity,” (1854) citing the First Epistle of John as Scriptural evidence:
*If you see your brother or sister committing what is not a mortal sin, you will ask, and God will give life to such a one—to those whose sin is not mortal. There is sin that is mortal; I do not say that you should pray about that. All wrongdoing is sin, but there is sin that is not mortal. * (1 John 5:16-17, NRSV)
But is this a correct rendering and interpretation of the Epistle? Even a note in the traditional Douay-Rheims states,
It is hard to determine what St. John here calls a sin which is not to death, and a sin which is unto death. The difference can not be the same as betwixt sins that are called venial and mortal: for he says, that if a man pray for his brother, who commits a sin that is not to death, life shall be given him: therefore such a one had before lost the life of grace, and been guilty of what is commonly called a mortal sin. And when he speaks of a sin that is unto death, and adds these words, for that I say not that any man ask, it cannot be supposed that St. John would say this of every mortal sin, but only of some heinous sins, which are very seldom remitted, because such sinners very seldom repent. By a sin therefore which is unto death, interpreters commonly understand a wilfull apostasy from the faith, and from the known truth, when a sinner, hardened by his own ingratitude, becomes deaf to all admonitions, will do nothing for himself, but runs on to a final impenitence.
It appears that this comment found in the Douay-Rheims (by Challoner himself?) is more in line with Pauline thinking than the actual Catechism. St. Paul does not divide the sins into mortal sins and venial sins. He simply says “sin” in general (“The wages of sin is death”). And the author of the Epistle of James, writing in the spirit of the true tradition of the Church, states,
But if you have respect to persons, you commit sin, being reproved by the law as transgressors. And whosoever shall keep the whole law, but offend in one point, is become guilty of all. (James 2:9, DRB)
Will not all sin eventually lead to spiritual death if not repented of? Why must there be a need for a distinction? The prophet Habukkuk even states plainly enough,
Your eyes are too pure to behold evil,
and you cannot look on wrongdoing (Habakkuk 1:13, NRSV)