Cardinal Manning gives examples of mortal sin: “The sin of Judas was a sin unto death. With his eyes open, with a knowledge of his Master,—though perhaps he did not know of the mystery of the Incarnation as we know it now; nevertheless he knew enough,—he sold his Master, and yet perhaps not knowing that he sold Him to be crucified. This, then, was a sin unto death. The sin of Simon Magus was a blasphemy and a sin unto death. The sin of those that blaspheme the Holy Ghost, which shall never be forgiven, is a sin unto death.”
As for “the sin of apostates from the faith, who, having known the truth, and having had the full light and illumination to know God, afterwards fall from Him, . . . ‘It is impossible for those who have been once enlightened, and have tasted of the Heavenly Gift, and of the good Word of God, and of the powers of the world to come, if they shall fall away, to be renewed again unto repentance’ (Heb 6:4-6). In one word, all who are impenitent sin unto death. . . . Saint John says, ‘They went out from us because they were not of us; for if they had been of us, without doubt they would have continued with us’ (1 Jn 2:19)—all these who so sin, sin unto death, and are left to the judgment of God.”
St. John distinguishes mortal sin (a sin unto death) from venial sin (a sin not unto death) when he writes: “If any man shall see his brother sin a sin which is not unto death, he shall ask, and life shall be given unto him that sinneth not unto death. There is a sin unto death: I do not say for that any man shall ask.” (1 Jn 5:16) Cardinal Manning comments: “Saint John in these words does not forbid us to pray [for the forgiveness of another person’s mortal sin]; he says, ‘I do not say’—that is, ‘I do not enjoin it.’ He leaves it to the conscience of every man. He says of those who sin not unto death, that ‘we have all confidence we may obtain pardon and grace for them;’ but for those who do sin unto death as I have described, ‘we have no such confidence, and therefore, though I do not enjoin it, I do not forbid it.'”
Quotations from Henry Edward Manning, Sin and Its Consequences, 2d ed. (London: Burns and Oates, 1874).