This is one of those passages that leaves commentators at odds. Opinions of what the “sin leading to death” is abound: blaspheming the Holy Spirit, any enormous sin (murder, idolatry, adultery), a sin punishable by death under Old Testament laws, a sin that required excommunication from the synagogue or the church, sins which brought fatal disease upon the offender, etc.
The Greek (ἁμαρτία πρὸς θάνατον) could be translated a sin which "tends" to death – a sin which would "terminate" in death or which carried the death penalty. The phrase could refer to a literal death of the body, spiritual death (e.g. dead in transgressions and sins Eph. 2:1), or eternal death in Hell. There could be an allusion to Jewish law where there was "a sin unto death" (חטאה למיתה) and "a sin not unto death" (חטאה לא למיתה). A sin unto death would be one to which the law prescribed the death penalty.
Of course the wages of even the smallest sin is eternal death, and no amount of good works or intercessory prayer will pay the debt. Only through the blood of Jesus and the grace of God are we forgiven for any sin, whether murder or stealing a pen from the bank.
One clue to understanding the passage may be found in the Greek. The Greek for "pray" signifies a request of an equal. And a Christian intercessor should not the authority in requesting forgiveness for a sinner who has sinned. The Greek word for "ask" implies the humble petition of an inferior. "Asking" for a brother sinning not unto death is a humble petition in consonance with God's will. To "request" for a sin unto death as if we were more merciful than God would be presumption.
I personally feel that the verse could indicate that we cannot pray someone into salvation, although we of course could pray that they come to saving faith. In Jesus’ high priestly prayer (Matthew 17) he interceded not for those in the world who refused to believe, but for those given to Him out of the world.It is our duty to pray for those who sin and to seek the salvation of believers who go astray, and in my opinion it is better to err in praying for all who are walking in sin rather than try to categorize what the sin leading to death may be.
Matthew Henry’s commentary puts it thusly - "We cannot pray that the sins of the impenitent and unbelieving should, while they are such, be forgiven them; or that mercy, which supposes the forgiveness of sins, should be granted to them, while they willfully continue such. But we may pray for their repentance, for their being enriched with faith in Christ, and thereupon for all other saving mercies. We should pray for others, as well as for ourselves, beseeching the Lord to pardon and recover the fallen, as well as to relieve the tempted and afflicted. And let us be truly thankful that no sin, of which any one truly repents, is unto death."