Let us be very clear about what 1 John 5:16 is saying (BLB):
If anyone should see his brother sinning a sin not unto death, he
shall ask, and He will give him life, to those sinning not unto death.
There is a sin unto death; I do not say that he should implore
concerning that.
"life" here is ζωή (zóé) which is define in this context a few verses earlier:
1 John 5:11, 12 - And this is that testimony: God has given us
eternal life, and this life is in His Son. Whoever has the Son has life; whoever does not have the Son of God does not have life.
This is the most common meaning of ζωή (zóé), namely, "eternal life" as per Matt 19:16, 29, 25:46, Mark 10:17, 25, 30, Luke 18:18, 30, John 3:15, 16, 36, 4:14, 36, 5:24, 39, 6:27, 40, 47, 54, 68, 10:28, 12:25, 50, 17:2, 3, Acts 13:46, 48, Rom 2:7, 5:21, 6:22, 23, Gal 6:8, 1 Tim 1:16, 6:12, Titus 1:2, 3:7, 1 John 1:2, 2:25, 3:15, 5:11, 13, 20, Jude 21.
The situation is best described by Benson in his comments on 1 John 5:16 -
1 John 5:16-17. If any man, &c. — As if he had said, Yea, he hears us
not only for ourselves, but others also; see his brother — That is,
any child of man; sin a sin which is not unto death — That is, any sin
but that which is marked out in the awful words of our Lord Jesus
Christ as unpardonable, namely, the blasphemy against the Holy Ghost,
of which see on Matthew 12:31; Mark 3:29. Or, which may rather be
intended, the sin of total apostacy from both the power and form of
godliness; he shall ask, and God shall give him life — Repentance unto
life, and, in consequence thereof, pardon and salvation for that
sinner. There is a sin unto death; I do not say that he shall pray for
it — That is, let him not pray for it. A sin unto death may likewise
mean one which God has determined to punish with temporal death. All
unrighteousness is sin — Every deviation from perfect holiness is sin;
but all sin is not unpardonable, nor does God determine to punish
every sin with temporal death.
That is, when a person sins, the sinners should seek divine pardon. Thus, as Benson correctly points out, most sins are not final, but some sins such as determined apostasy and the unpardonable sin.