'Sin leading to death' - what type of death?

The apostle seems to be speaking of physical death in this case. A deadly sin in this context refers to sins for which the scriptural punishment is death:

• Murder (Exodus 21:12, Leviticus 24:17, Numbers 35:16-21).

• Kidnapping (Exodus 21:16, Deuteronomy 24:7).

• Adultery (Leviticus 20:10, Deuteronomy 22:22-24).

• Worshiping other gods (Exodus 22:20, Deuteronomy 17:2-5).

• Breaking the Sabbath (Exodus 31:14, Numbers 15:32-36).

• Blasphemy (Leviticus 24:16).

• Human sacrifice (Leviticus 20:2).

John thus urges his readers to pray that sinners should not be punished by death, except for "deadly sins" such as the above. So he emphasizes that he does not request that Christians pray for deadly sinners to receive life, making the important distinction: "All wrongdoing is sin, but there is sin that is not deadly." (Whether John had all of these sins in mind is debatable, for Jesus himself has a liberal attitude toward Sabbath breaking and is known to have forgiven a woman taken in adultery.)

Later Christian tradition would speak of seven deadly sins in a more spiritual sense. These sins are Pride, Greed, Lust, Envy, Gluttony, Wrath and Sloth. However, it does not make sense that John was thinking along these lines - of spiritual death rather than literal death. He would ask for these sins, too, to be forgiven, for they are universal. They are among the very sins for which Christians are forgiven at baptism and which need to be forgiven again and again as they reassert themselves.

1 John 5:16-17 (NIV)

16 If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that. 17 All wrongdoing is sin, and there is sin that does not lead to death.

We may not need to determine the specific kind of sin that leads to death. The emphasis, rather, is on how an individual responds to their sins. Earlier in 1 John 1:9-10, John declares:

9 If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. 10 If we claim we have not sinned, we make him out to be a liar and his word is not in us.

John continues to build on this idea in 1 John 5:18:

We know that anyone born of God does not continue to sin; the One who was born of God keeps them safe, and the evil one cannot harm them.

Therefore, when one confesses their sins and turns away from them, those sins do not lead to death, for they are forgiven through God's faithfulness. Conversely, those who refuse to confess their sins make God out to be a liar, and their unrepented sins lead to death, for they have rejected the forgiveness He offers.

John's reference to death is clearly spiritual rather than physical in nature.

In Leviticus, there is a category of sin, presumptuous sin, that cannot be atoned by sacrifice. Then Jesus showed that intercessory prayer ideally has at its base the idea that the wrong doer doesn't know what he is doing in Luke 23:34. And Paul speaks about "if we sin willfully after we receive a knowledge of the truth, there remains no sacrifice for sin." So put these together and it is wilful presumptuous sin that leads to eternal loss until such time as personal repentance is made.