Tertullian

Touching this difference, we have not only already premised certain antithetical passages of the Scriptures, on one hand retaining, on the other remitting, sins; but John, too, will teach us: "If any knoweth his brother to be sinning a sin not unto death, he shall request, and life shall be given to him; "because he is not "sinning unto death," this will be remissible. "(There) is a sin unto death; not for this do I say that any is to request" -this will be irremissible.

For (in making these assertions) he was looking forward to the final clause of his letter, and for that (final clause) he was laying his preliminary bases; intending to say, in the end, more manifestly: "If any knoweth his brother to be sinning a sin not unto death, he shall make request, and the Lord shall give life to him who sinneth not unto death. For there is a sin unto death: not concerning that do I say that one should make request." He, too, (as I have been), was mindful that Jeremiah had been prohibited by God to deprecate (Him) on behalf of a people which was committing mortal sins.