We read about the “sin that leads to death” in 1 John 5:16. Note the context (i.e. 1 John 5:14-17):
14 And this is the confidence that we have toward Him, that if we ask anything according to His will He hears us. 15 And if we know that He hears us in whatever we ask, we know that we have the requests that we have asked of Him. 16 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. 17 All wrongdoing is sin, but there is sin that does not lead to death. (ESV)
This passage is the source of much debate. Many mistakenly attribute the “sin that leads to death” to the “unpardonable sin” (cf. Mark 3:22-30; Matt. 12:22-32). Some say it refers to sins that resulted in physical death, as in the deaths of Ananias & Sapphira (Acts 5:1-11). Still others believe John is referring to civil crimes in which the offender was sentenced to death. Catholics, under the doctrinal delusion of “purgatory,” argue that this refers to sins that might or might not be pardoned after death.
Yet, simply because this passage is debated does not mean we cannot be absolutely certain about what this passage teaches. The fact that baptism is a requirement for salvation and the reality that Christ has only one Church are also both highly debated, but this does not mean the Bible is not clear on these issues.
Guy N. Woods concisely clears up some of the confusion:
The “death” referred to was not bodily death, the loss of life; but, spiritual death, separation from God and all that is good. It was such a sin as only children of God may be guilty of. It is specifically declared to be the sin of a “brother.” Any correct interpretation must take into account these considerations and embrace them. The sin contemplated was not such as is usually designated as capital, i.e., such gross sins as idolatry, murder, adultery, blasphemy, etc. The effects thereof are visible and obvious. It is possible for one who prays to distinguish between the sin “unto death,” and the sin “not unto death.”
John is not referring to a specific sin, but rather a kind of sin. So to what kind of sin is the apostle referring?
John has much to say about sin in his epistle, making it unnecessary to go to another book to find the answer. In 1 John, we learn that we are all susceptible to sin (1 John 1:8; cf. Rom. 3:23). Yet, even though we sin, so long as we confess our sins (1 John 1:9) while actively demonstrate a penitent spirit (cf. Luke 18:9-14), the blood of Jesus continually cleanses us from all sin (1 John 1:7) and Christ advocates for us to the Father (1 John 2:1).
We know the Lord will forgive every sin a Christian confesses (1 John 1:8). Yet, there is a sin that the Lord will not forgive – namely, the “sin that leads to death” (1 John 5:16). Therefore, the “sin that leads to death” is simply a sin that a Christian refuses to repent of and confess.
We can confidently approach God in prayer, asking anything according to His Will (1 John 5:14-15). Yet, to pray that God will forgive a brother who has yet to repent and confess his fault is to ask God to do that which is against His Will.
Praying that God will forgive an impenitent brother would be just as pointless as asking God to forgive an alien sinner who has yet to repent and be baptized for the remission of sins (cf. Acts 2:38; 22:16, &c).
If your brother knows that you are guilty of sin, you are to confess your sin to your brother.
Therefore, confess your sins to one another and pray for one another, that you may be healed. The prayer of a righteous person has great power as it is working. (James 5:16, ESV)
In light of what we have discussed, the account of Simon in Acts 8:1-24 makes perfect sense. Simon, though he was a Christian, attempted to exploit the power of the Holy Spirit for selfish gain (Acts 8:18-19). As a result, his soul was in a lost state, and to be saved he was instructed to repent of his sins (verses 20-23). What did Simon do? He repented and confessed to Peter the sin which he had committed (verse 24).
To answer the question, “What is the sin that leads to death?” Guy N. Woods answers the question in short:
The “sin unto death” is therefore, a disposition of the heart, a perverseness of attitude, and an unwillingness of mind to acknowledge one’s sin, and from it turn away. This attitude effectively closes heaven’s door in one’s face!