27 “On the other hand, no one can enter a strong man’s house and rob his possessions unless he first ties up the strong man. Then he will rob his house. 28 I assure you: People will be forgiven for all sins and whatever blasphemies they may blaspheme. 29 But whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”— 30 because they were saying, “He has an unclean spirit.”
The strong man in Jesus’ illustration is Satan whose possessions are the people who are a slave to sin and under his authority. Someone stronger has to be the one to tie him up (Satan) and rescue them from his grasp, to set them free and bring them to God. Jesus was referring to Himself as being the stronger One.
14 Now since the children have flesh and blood in common, Jesus also shared in these, so that through His death He might destroy the one holding the power of death—that is, the Devil— 15 and free those who were held in slavery all their lives by the fear of death.
To blaspheme the Holy Spirit is to deny and reject the person of Jesus Christ and His deity. Since some were declaring that Jesus was being used by Beelzebul (Satan, the ruler of demons) they were rejecting the evidence presented to them by Jesus (miracles) that indeed the power of God was in their midst, God manifesting Himself in human form.