Mark 3:20-30 contains a familiar passage with a long debated phrase:
3:20 Now Jesus went home, and a crowd gathered so that they were not able to eat. 21 When his family heard this they went out to restrain him, for they said, “He is out of his mind.” 22 The experts in the law who came down from Jerusalem said, “He is possessed by Beelzebul,” and, “By the ruler of demons he casts out demons.” 23 So he called them and spoke to them in parables: “How can Satan cast out Satan? 24 If a kingdom is divided against itself, that kingdom will not be able to stand. 25 If a house is divided against itself, that house will not be able to stand. 26 And if Satan rises against himself and is divided, he is not able to stand and his end has come. 27 But no one is able to enter a strong man’s house and steal his property unless he first ties up the strong man. Then he can thoroughly plunder his house. 28 I tell you the truth, people will be forgiven for all sins, even all the blasphemies they utter. 29 But whoever blasphemes against the Holy Spirit will never be forgiven, but is guilty of an eternal sin” 30 (because they said, “He has an unclean spirit”). [NET Bible]
What has been the understanding of this "blasphemy against the Holy Spirit" and why is it unforgivable? Since other blasphemies will be forgiven (those against the Father and the Son), why will blasphemies against the Spirit not be forgiven?