Question:
Read 1 John 5:16. What is the sin unto death?

Answer:
Ananias and Sapphira sinned “a sin unto death” when they lied to the Holy Ghost by giving the impression they gave all the money when keeping part of it, and they died (Acts 5:1-5).

There was fornication and other sins in the assembly at Corinth (1 Corinthians 5:1-3), and “many are weak and sickly among you, and many sleep [die]” (1 Corinthians 11:30). It is important that each believer examines himself before eating of the Lord’s Supper (1 Corinthians 11:28). But not all sickness and death is God’s judgment for sin in our lives. We are not told what sin fits into the category of a “sin unto death,” and we could not say that another person has sinned a sin unto death, but there is a condition of heart which is unto death, and God knows what that condition is. It would be a sin which a believer commits that his testimony would be ruined; he would only be a hindrance to the testimony for Christ, and God could not trust him in this world any longer.