Concerning sin unto death, Bible teachers have different interpretations. Some say that it refers to the sin of the antichrists in denying that Jesus is the Christ (2:22), which keeps them in death forever. But according to the context of this verse, sin unto death is related to a sinning brother, not to an antichrist or any other unbeliever. Since this section, vv. 14-17, is related to prayer in the fellowship of eternal life (covered in 1:3--2:11), whatever it deals with must be related to the matter of the fellowship of the divine life. In the fellowship of the divine life there is the governmental dealing of God according to the spiritual condition of each of His children. In God's governmental dealing, some of His children may be destined to physical death in this age because of a certain sin, and others may be destined to physical death because of other sins.
The situation is like that of Ananias and his wife, Sapphira, who were dealt with by physical death because they lied to the Holy Spirit (Acts 5:1-11), and like that of the Corinthian believers, who were dealt with by the same judgment because they did not discern the Body (1 Cor. 11:29-30). This was typified by God's dealing with the children of Israel in the wilderness (1 Cor. 10:5-11). All the Israelites except Caleb and Joshua were judged by God with physical death because of certain sins. God's governmental dealing is severe. Miriam, Aaron, and even Moses were not spared from this kind of dealing, which came because of certain of their failures (Num. 12:1-15; 20:1, 12, 22-29; Deut. 1:37; 3:26-27; 32:48-52).
The punishment of God's governmental dealing with His children is not at all related to eternal perdition; it is a dispensational dealing according to the divine government, a dealing that is related to our fellowship with God and with one another. Whether or not a sin is unto death depends on God's judgment according to the sinning believer's position and condition in the house of God. In any case, for the children of God to sin is a serious matter. It may be judged by God with physical death in this age! The apostle did not say that we should make request concerning a sin that is unto death.
Thanks