1 Samuel 2:25

Because their father [Eli] did not chastise them with enough severity … he moved the forbearance of God to wrath so great that foreign peoples rose up against them and killed those sons of his in war in one day. His entire nation, furthermore, was vanquished, and a considerable number of his people fell.

The sin of one who blasphemes against the Holy Spirit is unpardonable. Compare with this judgment what is said in the book of Kings [Samuel]: "If one man shall sin against the Lord, who shall pray for him?" Thus, it is one and the same …

"Eli the priest," thou sayest, "speaketh, saying, If one man sin against another, they shall pray for him: but if a man sin against the Lord, who shall intreat for him? In like manner John, If any man see his brother sin a sin which …

19. For a man to sin against a man is for one human to do perverse things against another human. What then does it mean when he says, "God can be appeased for him," unless that such sins were light in comparison with those which the Jews were planning to commit against the Son of God? For they were sinning against God, because he whom they were plotting to kill had shown himself to be God by the most manifest light of miracles. For the Lord showed this through himself, saying: "If I had not done among them the works that no one else has done, they would have no sin" (John 15:24). And to show the implacability of God the Father, he adds, saying: "But now they have no excuse for their sin, because they have seen and hated both me and my Father" (ibid.). It is therefore as if he were saying: By whose prayer is that sin pardoned which is committed against the very one who pardons? Who beseeches the almighty Father on behalf of the one who condemns the coeternal Only-begotten to death? For he would say nothing more plainly if he were to declare openly, saying: This one whom you persecute as a mere man is also God. The law of reason therefore demands that he who persecutes the Creator be deprived of pardon. But divine mercy surpasses the strictness of reason; for he showed the equity of the law, but did not maintain the force of the pronounced declaration in the execution of the promulgated judgment. For by the rigor of equity it was decreed that no prayer would attain the obtaining of pardon; but for those for whom no prayer of man sufficed, the mercy of the Redeemer did not fail. Every man who might pray for the transgressions is removed; but for those for whom no man was sufficient, God himself became man, a merciful intercessor. For hanging on the cross he prayed, saying: "Father, forgive them, for they know not what they do" (Luke 23:34). […]

For a man to sin against a man is for people to harm one another in matters of passing things. But to sin against God is to take away from Him those things which He established for eternity. Therefore whoever draws either his own or another faithful person's soul into sinning is convicted of sinning against God, because he strives to take away from Him what is properly His. This is shown to be a sin all the more grave, inasmuch as one who might stand as a worthy intercessor for its absolution is more rarely found. For under the question "Who will pray for him?" it is not said that the remedies of repentance are denied even to such persons, but that the deeper wounds of sins are healed with greater difficulty. However, this can be understood to have been said because the priests are being rebuked. For a man sins against a man when he offends whose fault looks to the judgment of a superior person for correction. Therefore, since the sins of the subject people, which pertain to the priests, are wiped away by the prayers of those same priests, when the priest falls into fault, there is no superior person by whose prayers he may be expiated. And the Truth Himself intimates this, saying: "If the salt has lost its savor, with what shall it be salted?" (Matt. 5:13). Let him therefore say: "Who will pray for him?" As if to say, who remains as intercessor for him, when the one who was ordained to intercede for others casts himself down by transgressing? By this indeed the magnitude of the guilt is shown, because many of them are cast by the Lord into the darkness of an impenitent heart, and they do not come to their senses by any human exhortation....