I never said anything was false because it was legalistic. I also don’t see how it can be considered an “anti-legalistic” doctrine.
Is it not the belief that every sin fits neatly in one of two categories and the response we must make to the sin depends on the category it falls into?
I don’t think that this is legalism anymore than the distinction between sin and good works is legalism. There is a distinction of genus between venial and mortal sin, but these are not arbitrary categories. Rather, they take into account the object of the sin. Sins that direct one’s will against love of God or neighbor are generically mortal (remember the two greatest commandments?). Otherwise, they are not mortal (i.e. they are venial) generically.
For, when the will is directed to a thing that is in itself contrary to charity, whereby man is directed to his last end, the sin is mortal by reason of its object. Consequently it is a mortal sin generically, whether it be contrary to the love of God, e.g. blasphemy, perjury, and the like, or against the love of one’s neighbor, e.g. murder, adultery, and such like: wherefore such sins are mortal by reason of their genus. Sometimes, however, the sinner’s will is directed to a thing containing a certain inordinateness, but which is not contrary to the love of God and one’s neighbor, e.g. an idle word, excessive laughter, and so forth: and such sins are venial by reason of their genus. (I-II, 88, 2)
However, the nature of a sin is not just determined by the matter of the act, but by the disposition of the agent. Thus, a sin that is generically (we might also say materially) mortal can be venial or a sin generically venial mortal depending on the circumstances.
I hardly see how this is legalism. If anything, it is reading the law according to the spirit rather than the letter. I wonder what you propose we should say instead. Maybe you can clarify things by offering your own viewpoint. Do you believe that there is no division of sins into mortal and venial? Do you believe that the Catholic basis of division is not fitting?
We have to admit some mortal/venial distinction because we both acknowledge that Christians can fall from justice. If a sin causes us to fall from justice, then it is mortal. But not all sins immediately damn us to hell. These sins are not mortal. This is an objective distinction because either we are just or unjust in God’s eyes; there is no middle option. I hope you will give your own view so we can compare.