Let anyone blaspheme against the Holy Spirit and he will never have forgiveness. (Mark 3, 29)
The scribes who had come down from Jerusalem were saying, ‘Beelzebul is in him’ and, ‘It is through the prince of devils that he casts devils out.’ So he called them to him and spoke to them in parables, ‘How can Satan cast out Satan? If a kingdom is divided against itself, that kingdom cannot last. And if a household is divided against itself, that household can never stand. Now if Satan has rebelled against himself and is divided, he cannot stand either – it is the end of him. But no one can make his way into a strong man’s house and burgle his property unless he has tied up the strong man first. Only then can he burgle his house.‘I tell you solemnly, all men’s sins will be forgiven, and all their blasphemies; but let anyone blaspheme against the Holy Spirit and he will never have forgiveness: he is guilty of an eternal sin.’ This was because they were saying, ‘An unclean spirit is in him.’ (Mark 3,22-30)
Sins against the Holy Spirit are not remitted because they put up a wall to Christ's salvation. Every person is called to discover the truth that comes from God through reality, which never contradicts the truth. But if one insists on not recognising reality, they will never be able reach the truth and therefore God.