The Apostle John wrote that there is a sin that leads to death (1 John 5:16). Jesus referred to it as an unforgivable sin (Mark 3:28-29).
Christians have for years debated what that sin is. Some have thought it is murder or adultery; others have said suicide.
Here is the passage in 1 John 5:
If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. (1 John 5:16)