Some passages from Philip Melanchthon’s 1559 Loci Praecipui Theologici, The Chief Theological Topics, published by CPH in 2011. From Locus 11, The Difference Between Mortal and Venial Sin.
Mortal sin in a general sense is the defect or inclination or action contrary to the law of God, which offends Him, which is not forgiven, but merits the wrath of God and eternal punishment. (pg 229)
But because the person who is received when knowledge of Christ is kindled in his mind and faith in his heart and the Spirit given, and because the person also recognizes his weakness, deplores it, is ashamed at the knowledge of God’s wrath against his sins, seeks pardon, and fights back against the first flames of desire, for him these evils are venial sins, that is, they are forgiven, with the result that they do not drive out the Holy Spirit, and faith, and the person remains in a state of grace. Paul clearly teaches this in Rom. 8:1, “There is therefore no condemnation to those who are in Christ Jesus,” that is, even if sins are present in them, this weakness which I have mentioned, and evil desires, yet the person has been received by God and his condemnation taken away, as Paul says previously, “You are not under the Law, but under grace,” Rom 6:15. (pg 231).
Thus Paul also makes a distinction between the sins of those who remain in grace and those who reject grace, Rom. 8:13, “If you live according to the flesh, you shall die, but if through the Spirit you mortify the deeds of the body, you shall live,” that is, if your will gives in to depraved lusts and turns itself away from God knowingly and willingly is contrary to the commandment of God, refuses to repent, to fear God, and to believe Him, which is pleasing to God, your will does harm to your mind, so that it cannot call upon God, and with the loss of grace and Holy Spirit it again turns back to death, that is, it draws down upon itself the wrath of God and eternal punishment, just as Adam and Eve, when they fell, lost grace and brought upon themselves the terrible anger of God But if by the Spirit you mortify the deeds of the flesh, you shall live. He calls the depraved desires and negligence of our wicked nature, “the deeds of the flesh.” But then he continues by saying that we shall be victorious, that is, we shall retain grace and the Holy Spirit, if we contend against these wicked desires. This point is explained more clearly in other passages, where it is prescribed that there be in the regenerate the righteousness of a good conscience. 1 Tim. 1:5, “The purpose of the commandment is love that issues from a pure heart, a good conscience and sincere faith. (pg 231-232)
“For the weak will is captured even before it realizes it, but we must look to this fact (which can be easily understood and judged) so that we do not indulge in this knowingly but fight against it; and Paul adds in Rom 8:13 the words “by the Spirit,” that is, if we live with the true desire to repent, to fear and to love God and if our will is ashamed when it learns of the wrath of God, deplores is weakness, fears the traps of the devil, seeks pardon for the sake of Christ, asks also for the aid of the Holy Spirit, recalls the promises, and prepares itself as for a great battle, then it will not give way to these allurements but will vigorously repel them and will avoid opportunities for sin. The regenerate have the Holy Spirit, and they do not trouble Him by cultivating and kindling depraved desires, but understand that he Has been given to guide us and indeed He wants to guide us and help us, if we do not drive Him away but fight against our wicked desires. Therefore, it is said in 2 Cor. 6:1, “We beseech you that you do not receive the grace of God in vain.” David was able to keep the Holy Spirit and would have been aided by Him if he had not wanted to expel Him and wanted to give room to the burning lust which had arisen in his mind. (pg 232)