From J.W. Hanson’s Bible Threatenings Explained.
“If a man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin; and there is a sin unto death.” (1 John 5:16-17)
“The sin unto death” has often been supposed to be the “unpardonable sin,” so called, as though any sin could be unpardonable by a God whose mercy is without limit and without end. The apostle was merely alluding to the various offences under the Jewish law, some of which were unto death, or capital offences, while others were less heinous. The latter were to be interceded for, but the former were to be regarded as beyond intercession. On this passage Bishop Horne correctly says:
“The Talmudical writers have distinguished the capital punishments of the Jews into lesser deaths and such as were more grievous; but there is no warrant in the Scriptures for these distinctions, neither are these writers agreed among themselves what particular punishments are to be referred to these two heads. A capital crime generally was termed a sin of death (Deut. 16:6); or a sin worthy of death (Deut. 21:22), which mode of expression is adopted, or rather imitated, by the apostle John, who distinguishes between a sin unto death, and a sin not unto death (1 John 5:16). Criminals, or those who were deemed worthy of capital punishment, were called sons or men of death (1 Sam. 15:32; 1 Sam. 31:16; 2 Sam. 14:28, marginal reading), just as he who had incurred the punishment of scourging was designated a son of stripes (Deut. 25:16; 1 Kings 14:6). A similar phraseology was adopted by Jesus Christ, when he said to the Jews: “Ye shall die in your sins” (John 18:21-24). Eleven different sorts of capital punishment are mentioned in the sacred writings.”
Note: This chapter is taken from J.W. Hanson’s “Bible Threatenings Explained” from 1883. Some points made by the author may not be perfectly in sync with modern scholarship.