This is from “Believer’s Bible Commentary” by William MacDonald, page 2325:
THE SIN LEADING TO DEATH
It is impossible to say with finality just what sin leading to death is, and so perhaps the safest course to follow is to list various accepted interpretations and then tell which one we feel is most correct.
- Some feel that the sin leading to death refers to sin persisted in by a believer and unconfessed by him. In 1 Corinthians 11:30, we read that some had died because they partook of the Lord’s Supper without judging themselves.
- Others feel that the sin of murder is referred to. If a Christian should, in a moment of passion, murder another person, then we should not feel at liberty to pray for his release from the death penalty, because God has already stated that it is His will that “whoever sheds man’s blood, by man his blood shall be shed."
- Still others feel that the sin referred to here is blasphemy against the Holy Spirit. The Lord Jesus said that those who attributed His miracles which were done in the power of the Holy Spirit to Beelzebub, the prince of demons, had committed the unpardonable sin, and that there was no forgiveness for this sin either in that age or in the age to come.
- Others believe that it is some special form of sin such as that committed by Moses or Aaron, Ananias and Sapphira, and which God visits with summary judgment.
- A final explanation is that the sin of apostasy is in view, and we believe that this is the explanation which fits in best with the context. An apostate is one who has heard the great truths of the Christian Faith, has become intellectually convinced that Jesus is the Christ, has even made a profession of Christianity, although he has never been truly saved. After having tasted the good things of Christianity, he completely renounces them and repudiates the Lord Jesus Christ. In Hebrews 6 we learn that this is sin leading to death. Those committing this sin have no way of escape, since “they crucify again for themselves the Son of God, and put Him to an open shame."
In this entire Epistle, John has been speaking with the Gnostics in view. These false teachers had once been in the Christian fellowship. They had professed to be believers. They had known the facts of the faith, but then they had turned their backs on the Lord Jesus and accepted a teaching which completely denied His deity and the sufficiency of His atoning work. A Christian cannot have liberty in praying for the restoration of such because God has already indicated in His word that they have sinned unto death.
5:17 All unrighteousness is sin, and there is sin not leading to death.
There are distinct differences in the degrees of sin, and there are sins which are not of such a serious nature as to result in death.
My understanding is that we Catholics interpret “sin leading to death” as basically any sin that causes spiritual death, i.e., mortal sin. I’m struggling to discuss this with my Evangelical girlfriend, who essentially subscribes to #5 above - there’s apostasy, and there’s everything else; nothing short of apostasy can cause one to lose salvation.
Do understand the Catholic interpretation rightly? Are there other Scriptures that cover this territory? (That’d be helpful in talks with a sola scriptura adherent.)