The unforgivable sin is mentioned in the verses below.
Mt 12:31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
Mr 3:29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation:
Lu 12:10 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven.
It appears that John also commented on this in 1 John 5:16 – 17.
1Jo 5:16 If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
1Jo 5:17 All unrighteousness is sin: and there is a sin not unto death.
We can guess or try to draw conclusions about what this sin is or refer to scripture for the answer.
Heb 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
Willful sin is the sin which hath not forgiveness.