Isaiah 22:14
12 At that time the Sovereign Lord of Heaven’s Armies called for weeping and mourning, for shaved heads and sackcloth. 13 But look, there is outright celebration! You say, “Kill the ox and slaughter the sheep, eat meat and drink wine. Eat and drink, for tomorrow we die!” 14 The Lord of Heaven’s Armies told me this: “Certainly this sin will not be forgiven as long as you live,” says the Sovereign Lord of Heaven’s Armies.
Matthew 12:31
30 Whoever is not with me is against me, and whoever does not gather with me scatters. 31 For this reason I tell you, people will be forgiven for every sin and blasphemy, but the blasphemy against the Spirit will not be forgiven. 32 Whoever speaks a word against the Son of Man will be forgiven. But whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
Notes and References
"... The Holy Spirit here refers to the Jewish Spirit. As the Jews saw it, the Holy Spirit had two functions - to reveal God's truth and to enable men to recognise it. In this case, men had looked on the work of that Spirit but had not recognised it. They refused to accept that what they saw was the work of the Holy Spirit. The sin against the Holy Spirit was the refusing to follow its guidance. Forgiveness is impossible because, since the Holy Spirit can no longer be recognised, its guidance cannot be followed. Here men looked on good but saw evil. How then can repentance be possible if sin and evil cannot be recognised? Thus, this sin alone is unforgivable because by its very nature it precludes the possibility of repentance, the one condition which must be fulfilled before forgiveness can be offered and received. Some commentators see this saying as hyperbole. Jesus was only trying to show forcefully that blasphemy against the Holy Spirit was a far more terrible sin than that against men. This offers an apparently easy explanation but the OT passages cited in support (Numbers 15:3; 1 Samuel 3:14 and Isaiah 22:14) do not, in fact, seem to be entirely relevant. We seem to be left with the view of Vincent Taylor that the sin against the Holy Spirit 'is a perversion of spirit which, in defiance of moral values, elects to call light darkness'. In time, the moral values become upset and we are faced with the situation in which good has become evil and evil good. Repentance, and hence, forgiveness, are therefore impossible. Thus, it is not God, but man himself who brings about the possibility of an unforgivable sin. God's grace is still boundless, but it is man who cannot bring himself to avail of it ..."
Lochrie, John S. Forgiveness in the New Testament (pp. 43-44) The University of Glasgow, 1975