Augustine of Hippo

I would not have thee mistake that place in the epistle of John the apostle where he saith, "There are three witnesses: the Spirit, and the water, and the blood: and the three are one." Lest haply thou say that the Spirit and the water and the blood are diverse substances, and yet it is said, "the three are one:" for this cause I have admonished thee, that thou mistake not the matter. For these are mystical expressions, in which the point always to be considered is, not what the actual things are, but what they denote as signs: since they are signs of things, and what they are in their essence is one thing, what they are in their signification another. If then we understand the things signified, we do find these things to be of one substance. Thus, if we should say, the rock and the water are one, meaning by the Rock, Christ; by the water, the Holy Ghost: who doubts that rock and water are two different substances? yet because Christ and the Holy Spirit are of one and the same nature, therefore when one says, the rock and the water are one, this can be rightly taken in this behalf, that these two things of which the nature is diverse, are signs of other things of which the nature is one.

Three things then we know to have issued from the Body of the Lord when He hung upon the tree: first, the spirit: of which it is written, "And He bowed the head and gave up the spirit:" then, as His side was pierced by the spear, "blood and water." Which three things if we look at as they are in themselves, they are in substance several and distinct, and therefore they are not one. But if we will inquire into the things signified by these, there not unreasonably comes into our thoughts the Trinity itself, which is the One, Only, True, Supreme God, Father and Son and Holy Ghost, of whom it could most truly be said, "There are Three Witnesses, and the Three are One:" so that by the term Spirit we should understand God the Father to be signified; as indeed it was concerning the worshipping of Him that the Lord was speaking, when He said, "God is a Spirit:" by the term, blood, the Son; because "the Word was made flesh:" and by the term water, the Holy Ghost; as, when Jesus spake of the water which He would give to them that thirst, the evangelist saith, "But this said He of the Spirit which they that believed on Him were to receive."

This difference of sins it is that distinguishes Judas with his treason from Peter with his denial. Not that to him who repenteth there is to be no forgiveness: lest we go against that sentence of the Lord, in which He commands always to forgive the brother who asks his brother's forgiveness: but that the mischief of that sin is, that the man cannot submit to the humiliation of begging for pardon, even when he is forced by his evil conscience both to acknowledge and to publish his sin. For when Judas had said, "I have sinned, in that I have betrayed the innocent blood," he went and hanged himself in desperation, rather than pray for forgiveness in humiliation. Wherefore it makes a great difference, what sort of repentance God forgives. For many are much quicker than others to confess that they have sinned, and are angry with themselves in such sort that they vehemently wish they had not sinned, while yet they cannot lay down their pride, and submit to have the heart humbled and broken so as to implore pardon: a state of mind which one may well believe to be, for the greatness of their sin, a part of their already begun damnation.

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. But "a sin not unto death" is when a person, not having alienated his love from his brother, yet through some infirmity of mind may have failed to exhibit the due offices of brotherhood. Wherefore, on the one hand, the Lord on the cross said, "Father, forgive them, for they know not what they do," since they had not yet, by being made partakers of the grace of the Holy Spirit, entered into the fellowship of holy brotherhood; and blessed Stephen in the Acts of the Apostles prays for them who are stoning him; because they had not yet believed Christ, and were not fighting against that grace of communion. On the other hand, the apostle Paul does not pray for Alexander, and the reason I suppose, is, that this man was a brother, and had sinned "unto death," i.e. by opposing the brotherhood in a spirit of hatred.