OP: Q1: "What is the sin unto death?" (mortal?)
Q2: "What is the sin NOT unto death?" (venial?)
1 John 5:16-17 (NASB)
If anyone sees his brother committing a sin NOT leading to death (venial?),
he shall ask and God will for him give life to those who commit sin not leading to death.
There is a sin leading to death (mortal?); I do not say that he should make request for this.
All unrighteousness is sin, and there is a sin NOT leading to death.
1 John 5: 13-21
13 These things I have written to you who believe in the name of the Son of God,
so that you may know that you have eternal life.
14 This is the confidence which we have before Him, that, if we ask anything according to His will, He hears
15 And if we know that He hears us in whatever we ask, we know that we have the requests which we have asked from Him.
16 If anyone sees his brother committing a sin NOT leading to death,
he shall ask and God will for him give life to those who commit sin NOT leading to death.
There is a sin LEADING to death;
I do not say that he should make request for this.
17 All unrighteousness is sin, and there is a sin NOT leading to death.
18 We know that no one who is "born of God" sins;
but He who was born of God keeps him, and
the evil one does not touch him.
19 We know that we are of God, and that the whole world lies in the power of the evil one.
20 And we know that the Son of God has come, and
has given us understanding so that we may know Him who is true; and we are IN Him who is true, in His Son Jesus Christ. This is the true God and eternal life.
21 Little children, guard yourselves from idols.
RCC DOGMA!
http://catholicstraightanswers.com/what-is-the-difference-between-mortal-and-venial-sin/
Mortal versus Venial Sin
These two categories of sin are explicitly to be found in Sacred Scripture.
In the Old Covenant there were sins that merited the death penalty and sins that could be expiated by an offering.
This Law was a teacher that prepared the way for the faith (Gal. 3:24).
In the New Covenant these material categories are replaced by spiritual ones, natural death by eternal death.
There are thus daily faults for which we must daily ask forgiveness (Mt. 6:12),
for even the "just man falls seven times a day" (Prov. 24:16), and
mortal faults that separate the sinner from God (1 Cor.6:9-10) for all eternity.
1 Corinthians 6:9-10(NASB)
9 Or do you not know that the unrighteous will not inherit the kingdom of God?
Do not be deceived;
neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals,
10 nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers,
will inherit the kingdom of God.
The RELATIONSHIP between SIN and DEATH
Proverbs 10:16
The wages of the righteous is life, but
the earnings of the wicked are sin and death.
Romans 6:23
For the wages of sin is death, but
the gift of God is eternal life in Christ Jesus our Lord.
Romans 6:10
The death he died, he died to sin once FOR all;
but the life he lives, he lives to God.
Romans 8:2
because through Christ Jesus the law of the Spirit who gives life has set you free from the law of sin and death.
James 1:15
Then, after desire has conceived,
it gives birth to sin; and
sin, when it is full-grown, gives birth to death.
1 Peter 3:18 (NASB)...substitutionary atonement!
For Christ also died FOR sins once FOR all,
the just (Jesus) FOR the unjust,(Mankind)
so that He might bring us to God, (reconciliation/salvation)
having been put to death in the flesh,(Body/Soul bled to death)
but made alive in the spirit;(resurrection, appearances, ascension)