1 John 5:16-17 If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that. All unrighteousness is sin, and there is sin not leading to death.

My memory is all too familiar with this scripture.
It is a difficult passage to understand with so many diverse commentaries about it.
Above all remember this, although he is an Apostle, a follower of Jesus, John is Jewish. He was born Jewish and raised Jewish.
The Mosaic Law is ingrained into him.
Under the Law, there are penalties for sin that involved death and others that did not.
Adultery was punishable by death where eating pork was not, though adultery and eating pork are sinful under the Law.
A reminder, the Law was given not to make you perfect but to show you what sin is and that you are a sinner.(Romans 7:7)
God’s word declares in Ezekiel 18:4, “The soul who sins shall die” and in Romans 6:23, “the wages of sin is death.”
It is clear sin leads to death.
My question is this, is John talking a physical death or a spiritual death.
Perhaps a reminder of what the father said to his elder son about his younger son upon his return – It was right that we should make merry and be glad, for your brother was dead and is alive again, and was lost and is found. Luke 15:32
The prodigal son never lost his relationship as a son, despite leaving.
The father never stopped praying for the return of his son, who, according to the father, was dead to the family.
The son repented, returned, and was restored.
Back to our scripture, it’s my opinion there are sins that we do as Christians and are sorrowful for, repent, return, and are restored.
There are also sins that grab hold of us and our pride and pleasure keep us from repentance. That person dies spiritually and is on a slippery slope of backsliding from God.