Your comment: “1 John 1:9 doesn’t differentiate between “types” of sins. Nor does the NT.”
Again, you make false statements. John states that we need to walk in the light of the Lord in communion with others - the Body of Christ. If we are in darkness (mortal sin) we are not in the light(Truth) of God. Please note in Matt that “but whoever speaks against the Holy Spirit will not be forgiven”
Perhaps you should read and understand: John 5:16-18:
If anyone sees his brother committing a sin that is not a deadly sin, he will ask, and God will give him life for those whose sin is not deadly. There is sin which is deadly; I do not say one is to pray for that. All wrongdoing is sin, but there is sin which is not deadly. We know that anyone born of God does not sin, but He who is born of God keeps him, and the evil one does not touch him.
Three points:
1. These verses cannot be any plainer that there is such a thing as “deadly sin” and “sin which is not deadly.” That is precisely what the Church means by mortal (sin unto death) and venial (sin not unto death) sin.
2. St. John distinguishes the effects of mortal and venial sin as well. Members of the Body of Christ can pray for someone who commits venial sin (sin “which is not deadly”) and “life” (Gr. – zo-ay, or the divine life of God) and healing can be communicated to him through that prayer. But when it comes to “deadly sin,” St. John tells us not to “pray for that.” This is not meant to say we should not pray for a person in this state of sin at all. Scripture is very clear that we should pray for “all men” in I Tim. 2:1-2. The context seems to indicate that he is referring to praying that God “give [the wounded member of Christ] life” directly through that prayer. Divine life and healing can only come through members of the Body of Christ to other members in a direct way if the person being prayed for is in union with the Body of Christ. For mortal sin, one can only pray that God would grant the grace of repentance to the sinner so that they may be restored to communion with the Body of Christ through the sacrament of confession.
To understand this better, consider the analogy St. Paul uses for the people of God in I Corinthians 12:12-27—the analogy of the physical body of a human being. St. Paul tells us we are all members of “the Body of Christ.” A wounded finger that is still attached to its host body can be healed organically by the rest of the body. That kind of wound is analogous to the effect of venial sin. A severed finger, however, cannot be healed by the rest of the body because it is no longer attached to the body. That kind of wound is analogous to the effect of mortal sin. So it is in the Body of Christ.
3. Just after distinguishing between mortal (deadly) and venial (non-deadly) sins, St. John says “anyone born of God does not sin.” We know St. John could not be referring to all sin here because he already told us in I John 1:8: “If we say we have no sin, we deceive ourselves, and the truth is not in us.” Christians sin. It is clear from the context that St. John is referring to mortal sin here. If we sin mortally, we are cut off from the Body of Christ and are no longer in union with God. In that sense, the one who is in union with God cannot sin mortally. This is yet another clear distinction between mortal and venial sins in this text.
Mortal Sin Lists
We’ve already seen examples of “venial sins” in I John 5:16 and Matt. 5:19, but when it comes to mortal sin in Scripture, there are actually multiple lists of deadly or “mortal” sins in various places in Sacred Scripture. Our Lord himself provides us with several of them in Matthew 15:18-20, Revelation 21:8 and 22:15. St. Paul gives us the rest in Ephesians 5:3-7, Colossians 3:5-6, Galatians 5:19-21, and I Corinthians 6:9-11.
Any one of these biblical texts makes very clear that the biblical data is clearly in favor of mortal sins, but for brevity’s sake I will cite just one of them (Eph. 5:3-6):
But immorality and all impurity or covetousness must not even be named among you, as is fitting among saints. Let there be no filthiness, nor levity, which are not fitting; but instead let there be thanksgiving. Be sure of this, that no immoral or impure man, or one who is covetous (than is, an idolater), has any inheritance in the kingdom of Christ and of God. Let no one deceive you with empty words, for it is because of these things that the wrath of God comes upon the sons of disobedience. Therefore do not associate with them…
According to St. Paul, no matter how “born again,” “saved,” or whatever you think you are, if you commit these sins and you do not repent, you will not go to heaven. That is the essence of what “mortal sin” means.
Also:
Matt. 12:32:
And whoever says a word against the Son of man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come (Matthew 12:32).