What is “the blasphemy of the Holy Spirit” and why did Jesus say it was the “unforgiveable sin?” Can we commit “the unforgivable sin” today?
First, spanning out and considering a larger context, this incident (3:19-30) is preceded by Jesus’ confirmation that He is indeed claiming to be God, who alone can forgive sins, in direct response to a charge of blasphemy from the Scribes—if only at this juncture in their unexpressed thoughts (2:1-12). Now we see him charging them with blasphemy for openly ascribing the works of God to the works of demons, or “Satan” (the prince of evil spirits, or the adversary). So in Mark’s chronology we see Jesus’ identity progressively revealed, and along with this revelation we see the Jewish leaders becoming increasingly bolder in their accusations against him, and in their denial of his deity—which is their rejection of God’s revelation of His salvation to humankind in Christ.
Specifically to the question of why Jesus defines blasphemy as the “unforgiveable sin,” it is important to identify from the text what that “blasphemy” is. And we do have the definitive statement that Jesus is speaking against the “blasphemy” of those who had said, “he has an unclean spirit” (3:30). In other words, this unforgivable blasphemy—committed in a specific time and place by a specific group of people--was the denial of Jesus’ deity, and not simply by suggesting he was a mere man rather than “the Son of Man,” but with the added force of claiming the works he performed to prove his identity were in fact works he performed by the power of “Satan.”
I have occasionally heard people wonder, “what if I have committed the unforgivable sin?” It is important that we remember the time and place context of this story. Again, Jesus was speaking to a specific group of people, who had committed a specific offense—that of denying that His works, done physically in their presence, were the works of God, and ascribing those works instead to the works of “Satan.” This is not a position any of us are in today, historically speaking. Beyond this, our theology, and specifically our soteriology, will dictate our response to someone who fears they may be “unforgivable.” When our faith in the power of the cross assures us that “there is now no condemnation for those who are in Christ” (cf. Romans 8:1) we can comfort one another with this “good news.”
 First century Jews would have indeed understood from their prophets that Israel’s Savior would be none other than God. In fact, it is not possible to call Jesus “Savior” without also calling him “God” (cf. Isaiah 43:3; 45:21; Hosea 13:4).
 “Soteriology” refers to the doctrine of salvation, or our belief about how one becomes saved, and would also encompass whether we believe one could ever “lose salvation.”