1 John 5:16-17
"A sin which does not lead to death" is one that is genuinely repented of and for which forgiveness is available because the attitude of the sinner is meek and truly sorrowful. A person may have this attitude, yet still sin on occasion out of weakness, ignorance, bad judgment, or even inadvertently. Both greater and lesser sins can fall under this category. Earlier in the book, the same apostle writes:
If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. (I John 1:8-9)
Our genuine confession admits to God that we are guilty of breaking His law and seek to be cleared of it by Christ's sacrifice. This true repentance leads to a fierce desire not to sin and to building righteous character. God thus lifts the penalty of the second death, and once again, we, by His grace, are back on the road to salvation.
The sin that John calls a "sin leading to death" is what others know as "the unpardonable sin." Again, both greater and lesser sins can lead to the attitude that causes someone to commit an unforgivable sin. Such a sin is deeply reinforced by the attitude of the sinner—an attitude that denies Jesus Christ as Savior, that flagrantly hates his brother, and refuses to obey God's laws and statutes. Rebellion and defiance set this sin apart from others!
Martin G. Collins
Are Some Sins Worse Than Others?