What is the sin that leads to death, or the sin unto death, that John talks about?

TL;DR:

The “sin that leads to death” is a willful, hardened rejection of God that only unbelievers can commit, cutting them off from the source of eternal life. Believers may face serious consequences from persistent sin, but nothing can separate us from God’s love.

what does the bible say?

The “sin that leads to death” is a deliberate, unrepentant turning away from God, often after hearing and rejecting the truth of Christ, which leaves a person outside of salvation and cut off from the source of life. Therefore, there is only one sin that leads to eternal death: unbelief. A life of persistent sin may indicate such unbelief, regardless of whether the person calls himself a “Christian” (Matthew 7:21-23; 1 John 2:19). While believers cannot lose their salvation (John 10:28), persistent sin can still have severe consequences—such as physical harm, relational breakdown, or loss of ministry—and serves as a warning (Acts 5:1-10; 1 Corinthians 11:29-30). While daily struggles with sin are forgivable, hardened rebellion against God is eternally serious. Ultimately, John’s message calls both believers and those around them to recognize sin’s danger and continually turn to Christ for life, restoration, and protection from its destructive effects.

from the old testament

Some sins were treated with particular severity under the Law, resulting in death without atonement—for example, deliberate rebellion or blasphemy against God’s covenant (Numbers 15:30–31; Leviticus 24:16). Unintentional sins, which could be forgiven with sacrifice, were punished differently from high-handed, defiant sins, which brought condemnation.
Israel’s history shows that persistent, unrepentant sin—turning away from God to idolatry—invited judgment and death (Deuteronomy 13:6–10; Ezekiel 18:24).

from the new testament

First John 5:16 notes, "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." John distinguishes between sins believers can pray for forgiveness from and a deeper rejection of God that shows someone has no life in Him.
Paul also describes sin that leads to physical judgment (1 Corinthians 11:27–30). He wrote concerning the casual treatment of the Lord's Supper, "That is why many of you are weak and ill, and some have died” (1 Corinthians 11:30).
Similarly, the disrespect Ananias and Sapphira showed by lying to the Holy Spirit resulted in their deaths (Acts 5:1-11). No one particular sin automatically leads to spiritual death for believers. Instead, certain sins can lead to the physical death of a believer. Paul appeared to have this idea in mind in 1 Corinthians 5:4-5 where he commanded the Corinthians to disfellowship someone who was carrying on a sexual relationship with his stepmother. This disciplinary action was to preserve the man from spiritual death, which a later passage suggests was effective (2 Corinthians 2:6-11).
Hebrews warns against deliberate, continual sin after knowing the truth, saying there remains no sacrifice for such rebellion (Hebrews 10:26–27). Indeed, a life characterized by sin shows that a person is likely an unbeliever regardless of what he or she professes to be (1 John 2:19).

implications for today

The “sin that leads to death” in 1 John 5:16–17 is more than a stumble or weakness; it is a settled, hardened rejection of God, meaning only unbelievers can commit the sin unto death. In John’s context, it most likely refers to those who once heard the truth of Christ, even appeared part of the church but then willfully turned away and denied Him. This sin is “unto death” because it cuts a person off from the very source of life—Jesus Himself—leaving no forgiveness when someone persistently refuses to repent and receive His mercy.

For believers, nothing can take away our salvation, not even our sin, but we are warned that there are some sins that do lead to physical death. Though forgiven once we are saved, there are still real consequences to sin. For example, drugs can lead to an overdose and death. Or a Christian leader who consistently engages in embezzlement or abuse within a church or ministry can have serious consequences—losing their ministry, damaging others, and experiencing emotional, relational, or even legal “death” as God allows discipline to protect His people and uphold holiness. Persistent sexual sin can lead to disease and death. Persistent, unrepentant sin can have tangible, harmful effects in our present life, even when eternal salvation is secure.

At the same time, John is not saying ordinary sins we struggle with daily are unforgivable; those are covered when we confess and repent. Deadly sin is a deliberate, final rejection of Christ’s saving work or a willful persistence in sin that shows we want darkness more than light. This calls us to recognize our sin and turn to Jesus, who alone rescues us from death and secures eternal life for all who believe in Him.

We are called to live holy lives before God, seeking the good of other believers, and praying for one another during times of weakness so God would bring sinning believers to repentance and restored fellowship rather than judgment and death.

understand

The sin that leads to death is a deliberate, hardened rejection of God, committed only by unbelievers.
Believers cannot lose salvation, but persistent sin can have serious real-life consequences.
All sin besides rejection of Jesus is forgivable, and we are called to pray for forgiveness.

reflect

How aware are you of the ways persistent sin in your life could bring serious consequences even though your salvation is secure?
When have you been tempted to harden your heart toward God, and how can you turn back to Him in those moments?
How can you cultivate a habit of praying for fellow believers who struggle with sin rather than judging them?

engage

How do we discern the difference between daily struggles with sin and patterns of persistent, harmful sin among believers?
How can our prayers and accountability support others in turning away from sin and toward Christ?
How can we as believers respond when a brother or sister engages in persistent, harmful sin while still affirming their eternal salvation?