What is the only sin that cannot be forgiven?
ANSWER: Blasphemy against the Holy Spirit
Blasphemy Against the Holy Spirit – Matthew 12:22-32 ESV
“Then a demon-oppressed man who was blind and mute was brought to him, and he healed him, so that the man spoke and saw. 23 And all the people were amazed, and said, “Can this be the Son of David?” 24 But when the Pharisees heard it, they said, “It is only by Beelzebul, the prince of demons, that this man casts out demons.” 25 Knowing their thoughts, he said to them, “Every kingdom divided against itself is laid waste, and no city or house divided against itself will stand. 26 And if Satan casts out Satan, he is divided against himself.
How then will his kingdom stand? 27 And if I cast out demons by Beelzebul, by whom do your sons cast them out? Therefore they will be your judges. 28 But if it is by the Spirit of God that I cast out demons, then the kingdom of God has come upon you. 29 Or how can someone enter a strong man’s house and plunder his goods, unless he first binds the strong man?
Then indeed he may plunder his house. 30 Whoever is not with me is against me, and whoever does not gather with me scatters. 31 Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. 32 And 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.”