Augustine of Hippo, on 1John 5:16

“He that knoweth his brother to sin a sin which is not to death, let him ask, and life shall be given to him, who sinneth not to death. There is a sin unto death: for that I say not that any man ask.”

“For it manifestly shows that there are some "brethren" whom we are not commanded to pray for, whereas the Lord bids us pray even for our persecutors. Nor can this question be solved except we acknowledge, that there are some sins in brethren that are worse than the sin of enemies in persecuting. The "sin," therefore, of a brother, "unto death," I suppose to be when, after the acknowledging of God through the grace of our Lord Jesus Christ, one fights against the brotherhood, and is set on by the fire-brands of hatred against the very grace through which he was reconciled to God.”

Imported from an open dataset — not yet checked against the printed edition.