The following is an excerpt from The MacArthur New Testament Commentary on 1 John 5.

This is the confidence which we have before Him, that, if we ask anything according to His will, He hears us. And if we know that He hears us in whatever we ask, we know that we have the requests which we have asked from Him. If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this. All unrighteousness is sin, and there is a sin not leading to death. (1 John 5:14–17)

At first glance, verse 16 appears to introduce an abrupt change of subject. But upon further consideration, the connection of verses 16 and 17 to verses 14 and 15 becomes clear. By giving one important exception, John illustrates in a contrasting manner the extent of God’s promise to answer prayer. When a believer sees a brother (a real or professing believer) committing a sin not leading to death, the apostle writes, he shall ask and God will for him give life to those who commit sin not leading to death. On the other hand, there is a sin leading to death, and the apostle did not advise Christians to make request for this sin.

Evidently John and his readers knew what the sin leading to death was, since no explanation is given, but its exact meaning is difficult for us to determine. Two possibilities present themselves.

First, the sin in question may be that of a non-Christian leading to eternal death. In that case it would be a final rejection of Jesus Christ, such as that committed by those who attributed His miracles to the power of Satan (Matt. 12:31–32). Such ultimate apostasy is unforgivable, as Jesus declared:

Therefore I say to you, any sin and blasphemy shall be forgiven people, but blasphemy against the Spirit shall not be forgiven. Whoever speaks a word against the Son of Man, it shall be forgiven him; but whoever speaks against the Holy Spirit, it shall not be forgiven him, either in this age or in the age to come. (Matt. 12:31–32)

Praying for the restoration of such people to the fellowship from which they have departed (1 John 2:19) is futile, because “it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame” (Heb. 6:6). John did not forbid prayer for such people, since it is impossible to know who they are. The apostle merely stated that prayer for them will not be answered; God has already made the final decision about their future. Supporting the view that John is referring to unbelievers is the present tense of the participle hamartanonta (“sinning”; the Greek text literally reads “If anyone sees his brother sinning a sin …”); John elsewhere in this epistle uses the present tense to describe the habitual sins that characterize unbelievers (e.g., 3:4, 6, 8; 5:18).

Another possibility is that John is not referring to an unbeliever, but to a believer. According to this view, the sin leading to death refers to a Christian’s sin that is so serious that God takes the life of the one committing it. He put to death Ananias and Sapphira when they lied to the Holy Spirit in front of the church (Acts 5:1–11). Paul wrote to the Corinthians concerning those who were abusing the Lord’s Table, “For this reason many among you are weak and sick, and a number sleep [have died]” (1 Cor. 11:30). The sin is not one particular sin, but any sin that the Lord determines is serious enough to warrant such severe chastisement.

Both of the above views reflect biblical truth, and it is hard to be dogmatic as to which one John had in mind. In either case, John’s point is that prayer for those committing a sin leading to death will not result in the outcome that might otherwise be expected.

Although God mercifully does not immediately punish every sin with death, every sin is nonetheless a serious matter to Him. All unrighteousness is sin, John reminded his readers, even sin not leading to death. Every sin is a violation of His law and an affront to God, and is to be confessed (1:9; Ps. 32:5), forsaken (Prov. 28:13), and mortified (Rom. 8:13; Col. 3:5).