In his Commentary on the Heidelberg Catechism, following his commentary on Q&A #7, Zacharius Ursinus enters into an extended discussion of sin. He gives a basic definition and then makes several distinctions between varying kinds of sins. The first is the common distinction between “original sin” and “actual sin.” This is a distinction between that sin which we inherit from our first parents, our want of original righteousness and inclination towards evil, and that of the sins we ourselves actually commit, whether, Ursinus says, through a motion of the “understanding, will, and heart” or “the external deportment of our lives.”
Within this “actual sin,” however, there are still more distinctions to be made, one of which is the distinction between “reigning sin” and sin that is not reigning. Reigning sin is “that form of sin to which the sinner makes no resistance through the grace of the Holy Spirit.” Ursinus even says that this is the proper way to understand the distinction between mortal and venial sins. He writes:
Sin which does not thus reign, is that which the sinner resists by the grace of the Holy Spirit. It does not, therefore, expose him to eternal death, because he has repented and found favor through Christ. Such sins are disordered inclinations and unholy desires, a want of righteousness, and many sins of ignorance, of omission, and of infirmity, which remain in the godly as long as they continue in this life; but which they, nevertheless, acknowledge, deplore, hate, resist, and earnestly pray may be forgiven them for the sake of Christ, the Mediator, saying, forgive us our debts. Hence the godly retain their faith and consolation, notwithstanding they are not free from these sins. “If we say we have no sin, we deceive ourselves, and the truth is not in us.” “It is no more I that do it, but sin that dwelleth in me.” “There is no condemnation to them that are in Christ Jesus, who walk after the Spirit.” “Who can understand his errors? Cleanse thou me from secret faults.” (1 John 1:8. Rom. 7:18; 8:1. Ps. 19:13.)
The common distinction of sin into mortal and venial may be referred to this division. For although every sin in its own nature is mortal, by which we mean, that it deserves eternal death, yet reigning sin may be properly so called, inasmuch as he who perseveres in it will at length be overtaken by destruction. But it becomes venial sin that is, it does not call for eternal death, when it does not reign in the regenerate who resist it by the grace of God; and this takes place, not because it merits pardon in itself, or does not deserve punishment, but because it is freely forgiven those that believe on account of the satisfaction of Christ, and is not imputed to them unto condemnation, as it is said: “There is no condemnation to them that are in Christ Jesus.” (Rom. 8: 1.) When thus understood, the distinction of mortal and venial sin may be retained; but not when it is understood in the sense in which the Romish priests use it, as if that were mortal sin which deserves eternal death on account of its greatness, and that venial which does not deserve eternal death on account of its smallness, but merely some temporal punishment. Hence we would prefer, in the place of mortal and venial sin, the distinction which we have made of sin into reigning, and not reigning, and that for the following reasons:
1. Because the terms mortal and venial are ambiguous and obscure. All sins are mortal in their own nature. The apostle John also calls the sin against the Holy Ghost mortal, or unto death.
2. Because the Scriptures do not use these terms, especially venial sin.
3. Because of the errors of the Papists, who call those sins venial which are small and do not deserve eternal death, whilst the Scriptures declare: “Cursed be he that confirmeth not all the words of this law to do them.” “Whosoever shall offend in one point, is guilty of all.” “The wages of sin is death.” “Whoso shall break one of these commandments, and shall teach men so, he shall be called the least in the kingdom of God.” (Deut. 27: 26. James 2: 10. Rom. 6: 23. Matt. 5: 19.) In a word, every sin in its own nature is mortal, and deserves eternal death. But it becomes venial, that is, it does not work eternal death in the regenerate, because their sins have been freely pardoned for the sake of Christ.
(Commentary on the Heidelberg Catechism, P&R reprint of the 1852 ed, pg. 45)