Image via WikipediaI agree that we need to understand what God says in one part of the Bible in the context of what he says to us in the rest of the Bible when considering individual passages. However, a Scripture in isolation also has to make sense in its own right and in its immediate context, which is why I made the points that I did. If there's an apparent contradiction between passages then there must be a solution which makes sense of both. I think that your interpretation doesn't do justice to 1 Jn 5:16-17; I don't see any indication in the passage that he is referring to forgiven sin and, in its immediate context, 1:9 ("If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.") and 5:13 ("I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life.") seem, taken together, to explicitly contradict it.
One of the reasons that I became a Catholic was because I was impressed at the coherence of the Church's teaching - even on controversial topics like faith and works - passages that seemed to contradict each other seemed endlessly to be explained, rather than being explained away or ignored, as you might suspect. The Church certainly makes a big deal about the importance of rigorous study of Scripture.
I don't see that the rest of the bible does contradict distinction between sins that do and don't lead to death and therefore think that John's teaching should be taken at face value. Scripture does indicate distinctions between the seriousness of various sins:
Luke 12:47-48
47"That servant who knows his master's will and does not get ready or does not do what his master wants will be beaten with many blows. 48But the one who does not know and does things deserving punishment will be beaten with few blows. From everyone who has been given much, much will be demanded; and from the one who has been entrusted with much, much more will be asked.
John 19:11
11Jesus answered, "You would have no power over me if it were not given to you from above. Therefore the one who handed me over to you is guilty of a greater sin."
Acts 17:30
30In the past God overlooked such ignorance, but now he commands all people everywhere to repent.
1 Timothy 1:13
13Even though I was once a blasphemer and a persecutor and a violent man, I was shown mercy because I acted in ignorance and unbelief.
Hebrews 10:26and that there are sins that will cost a believer their salvation:
26If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left
1 Corinthians 6:9-10 cf. 5:1 & 1:2
9Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders 10nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God.
Matthew 5:21-22
21"You have heard that it was said to the people long ago, 'Do not murder, and anyone who murders will be subject to judgment.' 22But I tell you that anyone who is angry with his brother will be subject to judgment. Again, anyone who says to his brother, 'Raca,' is answerable to the Sanhedrin. But anyone who says, 'You fool!' will be in danger of the fire of hell.
Ephesians 5:3-5 c.f. 1:1
3But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God's holy people. 4Nor should there be obscenity, foolish talk or coarse joking, which are out of place, but rather thanksgiving. 5For of this you can be sure: No immoral, impure or greedy person - such a man is an idolater - has any inheritance in the kingdom of Christ and of God.
James 1:13-15 c.f. 1:2For a Catholic, there's no need to tie oneself in knots over the interpretation of Scripture, as both the interpretation of Scripture and forgiveness belong to the whole Church (1 Tim 3:15, Mt 10:40,16:17-19,18:17-18, Jn 20:21-23) rather to any individual in the one body of Christ.
13When tempted, no one should say, "God is tempting me." For God cannot be tempted by evil, nor does he tempt anyone; 14but each one is tempted when, by his own evil desire, he is dragged away and enticed. 15Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.
For Catholics, mortal sin is defined by the Church as involving grave matter (along the lines of the ten commandments, but defined explicitly by the Church in some cases - Mt 19:16-19 c.f. 5:17-20), sufficient knowledge and reflection (Lk 12:47-48, Acts 17:30, 1 Tm 1:13) and full consent of the will e.g. not at gunpoint (Hb 10:26). This kind of sin, we teach, kills the life of grace. Other (venial) sin is no laughing matter though; it's like the difference between having a fever and being a corpse. This, to me, makes sense of both "the wages of sin is death" (Rm 6:23) and "there is sin which does not lead to death" (1 Jn 5:17), especially as Jm 1:15 speaks of death as the consequence of sin "when it is full-grown". Venial sin might be said to lead to death in a different sense to mortal sin, as a man might die through a series of injuries rather than one fatal blow.
In practical/spiritual terms, I don't think it's as problematic as you suggest that it might be. I remember having more agonising doubts and fears over my sin before I became a Catholic, especially worrying about the sincerity of my repentance. In any case, the best thing to do with all sin is to confess it; you might think a sin is more or less serious, but you need any kind of sin like a hole in your head, and we know where to go for forgiveness, so, in practical terms, it's not something to agonise over unless you happen to like agony.
