"If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that." 1 John 5:16 [ESV]
This passage contains important information for us, but we cannot act upon it. For we can say of no person before his death that he has committed the sin against the Holy Spirit. [...] This is a shocking statement, and yet it contains a great comfort. Some one may come to you and say: 'I am a wretched man - I have committed the sin against the Holy Ghost. I am quite certain of it.' The afflicted may well tell you of the evil he has done, the evil he has spoken, and the evil he has thought. It may really look as if he had blasphemed the Holy Ghost. Now remember the weapon which Heb. 6 furnishes for attacking a case like this: That person is not at all rejoicing over what he tells you; it is all so awfully horrid to him. This shows that God has at least begun to lead him to repentance; all that he need do is to lay hold of the promise of the Gospel. [...] A case like this is indeed not to be treated lightly; the sufferer must be shown that, since there is in him the beginning of repentance, he has an indubitable proof that he has not committed the sin against the Holy Ghost. In general, when preaching on this subject, the minister must aim at convincing his hearers that they have not committed this sin rather than warn them not to commit it. To a person who has really committed this sin preaching is of no benefit. Whoever is sorry for his sins and craves forgiveness should be told that he is a dear child of God, but is passing through a terrible tribulation.

--- C.F.W. Walther, "Thirty-Eighth Evening Lecture (October 23, 1885)", The Proper Distinction Between Law and Gospel