What does Jesus mean by eternally unforgiveable sin? Can this sin be committed today? By a believer?

And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. (Matthew 12 :32)

Some have read these words of Christ and surmised that “blaspheming the Holy Spirit” is either a sin the God cannot forgive, or one that God will not forgive. In both of these instances, God is left either unable to forgive and thus not omnipotent, or unwilling to forgive and thus not all loving.  

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 However, these interpretations not only do violence to Christian theology, but to the original historical context in which Christ spoke these words. In Matthew and Mark, the context is: Christ drives out a demon by the power of the Holy Spirit, which the Pharisees ascribe to Satan, to which Christ responds by first issuing this stern warning before reiterating His common analogy of recognizing trees by their fruit. Jesus response clarifies that there is no “middle ground” (also Mark 9:40) between the Kingdom of Heaven and the Kingdom of Satan in that blasphemy against the Holy Spirit indicates a heart condition of “complete perversion of spiritual values, revealing a decisive choice of the wrong side in the battle between good and evil, between God and Satan” (France, 482). This is precisely what the Pharisees had done by attributing the Holy Spirit's work to His ultimate enemy, and Christ called their diametrical opposition to the good purposes of God as unforgiveable (France, 482). This is made clear in the following verses, for if this is the fruit of their lives then they are not believers in Christ and are thus not forgiven. Put another way, “Jesus sees no hope for these people, because their moral vision is so confused and reversed as to be beyond remedy” (Filson, 150). 
While this particular context has passed, the principle behind this passage still applies to us today, but not as broadly as people fear. In the words of France, “this saying is a wake-up call to the arrogant, not a bogey to frighten those of tender conscience” (France 483), in that any Christian who is afraid they may have blasphemed the Holy Spirit confirms by their fear that they haven't. However, if the fruit of unrepentance is evident in a “Christian's” life, they would do well to verify that they have indeed been regenerated by the gospel for “a person in any age can reject the gospel with such determined resistance that they pass a point of no return” (Howell, 134). These, wolf-in-sheep's-clothing Christians are the ones that Hebrews (6:4-6; 10:26-31) and John (1 John 2:19; 5:16; 2 John 9) warned believers about. For once saved always saved (Perseverance of the Saints), but not all who think they're saved actually are (Matthew 7:23). Far from questioning the assurance of a believer's salvation, “this limit is not placed on God's ability [or willingness] to forgive, but on the capacity of a person to repent” (Howell, 134).

• Filson, Floyd V., Th.D.. A Commentary on the Gospel According to St. Matthew. London: Adam & Charles Black. 1960.
  
• France, R.T.. The New International Commentary on the New Testament: The Gospel of Matthew. Grand Rapids: Eerdmans Publishing Co. 2007.
  
• Howell Jr., Don N.. The Passion of the Servant. Eugene: Wipf & Stock - Resource Publications. 2009.