In the Bible blasphemy against the Holy Spirit is mentioned as an unforgivable sin. This has led to a number of suggestions as to what this unforgivable sin was - suicide, being involved in ‘satan worship’, adultery, listening to ABBA etc etc. However, these suggestions don’t fit in with what the Bible teaches.
Jesus speaks of blasphemy of the Holy Spirit in the Bible in Mark 3:22-30 [and in similar passages in Matthew 12:22-32 & Luke 12:8-12].
In these situations Jesus was warning others against the attitude that some people had to Jesus and what he was doing. It appears that not being able to fault his teaching, or dismiss his powerful miracles as fake, these people were attributing Jesus’ authority to being an agent of Evil (Satan, the Devil). It seems to me that behind this is a hard hearted refusal to recognise Jesus; a deep personal rejection of Jesus that scrabbles for any excuse; an entrenched, unfalsifiable skepticism. It’s easy to see how such an attitude, if left unchecked, will result in a lifelong rejection of Jesus,
This then is consistent with the rest of the Bible’s teaching - that Jesus’ death and resurrection is sufficient to cover any sin, so that whoever prays to Jesus asking for forgiveness and commits themselves to following Jesus will be forgiven. But also that whoever resists the promptings of the Holy Spirit to recognise Jesus as God’s King and refuses to humble themselves under Jesus, will not receive forgiveness.
John 3:36 puts it this way:
Everyone who has faith in the Son [Jesus] has eternal life. But no one who rejects him will ever share in that life, and God will be angry with them forever.