St John the Evangelist, by Vicente Juan Masip (1507-1579) [public domain / Wikimedia Commons]
[1997; additions on 2-25-03]
1 John 5:17 explicitly differentiates a mortal sin from a less serious one (RSV): “All wrongdoing is sin, but there is sin which is not mortal” (RSV; KJV: “not unto death”).
Denying this would be tantamount to saying that a white lie or a momentary pang of jealousy or lust (especially if unrepented of) is the moral equivalent in God’s eyes of a torture, rape, and murder? That’s what the Protestant position — starkly put — reduces to.
Everyone agrees that all sin is barred from heaven, but again, that is future for us, not present, and this is precisely why purgatory is such a merciful, necessary doctrine. If God gets “serious” about actual, real sin in heaven, why in the world would He not start now? Catholics think that God — in practical terms — takes sin as seriously now as He will then, and that’s one reason why we think mere imputation or forensic declaration of holiness is a falsehood.
1 John 5:16-17 expressly contradicts Protestant teaching on this. John says “he shall give him life for them that sin not unto death”, but Protestants say “all sin leads to spiritual death,” and that all sins are equal in God’s eyes. Who am I to believe? Again, the Apostle John says, “there is a sin not unto death” in 5:17. Thus he is clearly making the distinctions we make with regard to degrees of sin. Furthermore, it is not by any means certain from context that the “mortal sin” is blasphemy against the Holy Spirit alone. Some translations even have “there is sin,” rather than “there is a sin.”
Lastly, there are several lists of sins that are said to bar one from “the kingdom of heaven” (RSV):
1 Corinthians 6:9-10 Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither the immoral, nor idolaters, nor adulterers, nor sexual perverts,  nor thieves, nor the greedy, nor drunkards, nor revilers, nor robbers will inherit the kingdom of God.
Galatians 5:19-21 Now the works of the flesh are plain: fornication, impurity, licentiousness,  idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit,  envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God.
Ephesians 5:5 Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God.
Meta Description: One of the classic Protestant-Catholic differences regarding the theology of salvation is briefly discussed.
Meta Keywords: Catholic soteriology, co-laborers with God, cooperation with God’s grace, degrees of sin, faith, Faith Alone, grace, grace alone, heaven, hope, imputed justification, infused justification, Justification, merit, moral assurance of salvation, Mortal sin, perseverance, Salvation, sanctification, sin, soteriology, synergy, venial sin