There it was clear that, for Hemmingsen, all sins are mortal by nature (sua natura), and so, for the distinction to be useful, it must be recast as having reference, not to an objective difference between sins (though in other respects such differences do exist), but to a subjective difference between sinners: the sins of saints (that is, of Christians) can be called “venial” because they meet with the mercy (venia) of God, who does not exact the punishment that is their due from those who have been clothed in the righteousness of Christ.
At the end of his discussion of the Moral Law in Institutes 2.8, Calvin makes much the same point, 1 though in a very different rhetorical mode.
For Calvin, all sin is lawlessness; whoever breaks the Law in any particular is guilty of all of it, and thus is deserving of death. Thus “all sin is mortal, because it is rebellion against the will of God.” This sentence should be lingered over, and pondered with a more than ordinary seriousness.
Nevertheless, the sins of “the saints,” that is, of those who have put on Christ, are “venial,” for “they obtain pardon” due to the mercy of God in Christ. Though they were rebels, and “by nature children of wrath,” the saints–again, those who are in Christ–find remission for their sins in the divine covering.
The proper course had been to consider not simply what is commanded, but who it is that commands, because every least transgression of his Law derogates from his authority. Do they count it a small matter to insult the majesty of God in any one respect? Again, since God has explained his will in the Law, every thing contrary to the Law is displeasing to him. Will they feign that the wrath of God is so disarmed that the punishment of death will not forthwith follow upon it? He has declared plainly (if they could be induced to listen to his voice, instead of darkening his clear truth by their insipid subtleties), “The soul that sinneth it shall die,” (Ezek. 18:20). Again, in the passage lately quoted, “The wages of sin is death.” What these men acknowledge to be sin, because they are unable to deny it, they contend is not mortal. Having already indulged this madness too long, let them learn to repent; or, if they persist in their infatuation, taking no further notice of them, let the children of God remember that all sin is mortal, because it is rebellion against the will of God, and necessarily provokes his anger; and because it is a violation of the Law, against every violation of which, without exception, the judgment of God has been pronounced. The faults of the saints are indeed venial, not, however, in their own nature, but because, through the mercy of God, they obtain pardon. (Institutes 2.8.59)