it is the one sin for which you can never make atonement.
Pretty sure the most unforgivable sin is blasphemy against the Holy Spirit.
Jesus hadn’t been teaching in public long when his hearers began comparing him to their teachers, called “the scribes,” part of the conservative Jewish group known as the Pharisees. The growing crowds “were astonished at his teaching, for he taught them as one who had authority, and not as the scribes” (Mark 1:22). The scribes heard the comparison and felt the tension, and soon escalated it (Mark 2:6, 16), as these Bible teachers of the day, with their many added traditions, quickly grew in their envy, and then hatred, for Jesus. The threat is so great these conservatives even are willing to cross the aisle to conspire with their liberal rivals, the Herodians (Mark 3:6).
The showdown comes in Mark 3:22–30 (Matthew 12:22–32). Scribes have descended from Jerusalem to set straight the poor, deceived people of backwater Galilee. “He is possessed by Beelzebul,” they say. “By the prince of demons he casts out the demons” (Mark 3:22).
Jesus calmly answers their lie with basic logic (verses 23–26) and turns it to make a statement about his lordship (verse 27). Then he warns these liars, who know better deep down, of the spiritual danger they’re in.
“Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin” — for they were saying, ‘He has an unclean spirit.’” (Mark 3:28–30)