A.T. Robertson Commentary 1 John 5:16

"If any man see his brother sinning a sin not unto death, he shall ask, and [God] will give him life for them that sin not unto death. There is a sin unto death: not concerning this do I say that he should make request." — 1 John 5:16 (ASV)

If any man see (εαν τις ιδη). Third-class condition with εαν and second aorist active subjunctive of ειδον (οραω).

Sinning a sin (αμαρτανοντα αμαρτιαν). Present active predicate (supplementary) participle agreeing with αδελφον and with cognate accusative αμαρτιαν.

Not unto death (μη προς θανατον). Repeated again with αμαρτανουσιν and in contrast with αμαρτια προς θανατον (sin unto death). Most sins are not mortal sins, but clearly John conceives of a sin that is deadly enough to be called "unto death." This distinction is common in the rabbinic writings and in Nu 18:22 the LXX has λαβειν αμαρτιαν θανατηφορον "to incur a death-bearing sin" as many crimes then and now bear the death penalty. There is a distinction in Heb 10:26 between sinning wilfully after full knowledge and sins of ignorance (Hebrews 5:2). Jesus spoke of the unpardonable sin (Matthew 12:32; Luke 12:10), which was attributing to the devil the manifest work of the Holy Spirit. It is possible that John has this idea in mind when he applies it to those who reject Jesus Christ as God's Son and set themselves up as antichrists.

Concerning this (περ εκεινης). This sin unto death.

That he should make request (ινα ερωτηση). Sub-final use of ινα with the first aorist active subjunctive of ερωταω, used here as in Joh 17:15,20 (and often) for request rather than for question. John does not forbid praying for such cases; he simply does not command prayer for them. He leaves them to God.