Commentaries:
Robertson's Word Pictures (NT)
If any man see (ean tiv idh). Third-class condition with ean and second aorist active subjunctive of eidon (oraw).
Sinning a sin (amartanonta amartian). Present active predicate (supplementary) participle agreeing with adelfon and with cognate accusative amartian.
Not unto death (mh prov qanaton). Repeated again with amartanousin and in contrast with amartia prov qanaton (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 Numbers 18:22 the LXX has labein amartian qanathforon "to incur a death-bearing sin" as many crimes then and now bear the death penalty. There is a distinction in Hebrews 10:26 between sinning wilfully after full knowledge and sins of ignorance (Hebrews 5:2). Jesus spoke of the unpardonable sin (Mark 3:29; 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 (peri ekeinhv). This sin unto death.
That he should make request (ina erwthsh). Sub-final use of ina with the first aorist active subjunctive of erwtaw, used here as in John 17:15, John 17:20 (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.
Other Robertson's Word Pictures (NT) entries containing 1 John 5:16:
John 11:4
2 John 1:5
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