How do Protestants understand the "unforgivable" sin? Scripture refers to an unpardonable sin in Matthew 12:31–32, Mark 3:29–30, and Luke 12:10, as well as Hebrews 6:4–6.

There's also "sin that leads to death" (1 John 5:16–17).

Given the Protestant understanding of sin, what is an overview of the doctrines they develop from these passages?  To Protestants, what is the unforgivable sin?

Related, but not tradition-specific: What is the blasphemy of the Holy Spirit?
Protestants basically fall into three main camps, claiming that the unpardonable sin is: 


attributing the works of the Holy Spirit to the devil; or, 
refusal to repent even to the end of one's life; or,
hatefully and willfully slandering the Holy Spirit's testimony of Christ.  


Within (3), there are three views regarding who can commit the sin: 3a) only the regenerate (saved), 3b) both regenerate and unregenerate (unsaved), and 3c) only the unregenerate.

(1) Some, particularly dispensationalists like Lewis S. Chafer, take the view (also held by Jerome and Chrysostom) that the unpardonable sin could only be performed during Jesus' time on earth, since the sin specifically "consisted in asserting that Christ’s works, which were wrought by the Holy Spirit, were accomplished on the contrary by Satan."1  Charismatics like J. Rodman Williams similarly view it as the "utterly perverse ascription of the work of God to Satan," but argue that it can still be committed today.2

(2) The second position was held by Augustine,3 some early Lutherans, and several Scottish theologians.  One of the latter was Thomas Chalmers, who writes that "it is a sin which can be charged upon every man who has put the overtures of forgiveness away from him."  Expressing his disagreement with other views, he writes:


  So that this sin, looked upon by many as the sin of one particular age, or, if possible to realise it in the present day, as only to be met with in a few solitary instances of enourmous and unexpiable transgression, is the very sin upon which may be made to turn the condemnation and the ruin of the existing majority of our species.4


(3) The third position is held by later Lutherans, Arminians, and Calvinists.  The Lutherans espouse the first subdivision, the Arminians the second, and Calvinists the third.

(3a) Lutheran John Mueller explains that the sin is "committed only when the Holy Spirit has clearly revealed the divine truth to the sinner and the sinner nevertheless utters blasphemies against it."5  He notes that most dogmaticians6 in his tradition "teach that the sin against the Holy Ghost can be committed only by those who were regernerated," with the exception of those who argue that it can also occur in the unregenerate "in the very moment when the Holy Ghost is about to convert them."

(3b) The second subdivision, that either regenerate or unregenerate can commit the sin, is held by Arminians.  Jacobus Arminius described the sin as the "rejection and refusing of Jesus Christ through determined malice and hatred against Christ [...] against conscience."7

(3c) The third subdivision (only the unregenerate commit the sin) is believed by those in the reformed tradition, like John Calvin and Louis Berkhof. Calvin writes that the sin is committed by those "whose consciences, though convinced that what they repudiate and impugn is the Word of God, yet cease not to impugn it,"8 but that "he who has been truly regenerated by the Spirit cannot possibly fall into so horrid a crime."9 Berkhof's definition reads:


  The sin consists in the conscious, malicious, and wilful rejection and slandering, against evidence and conviction, of the testimony of the Holy Spirit respecting the grace of God in Christ, attributing it out of hatred and enmity to the prince of darkness.10




Notes and References: Berkhof and Combs provide helpful summaries; Combs in particular includes additional quotes, including some from church fathers and medieval theologians, and a review of how modern theologians fall into the camps above.


Combs, William W., "The Blasphemy Against the Holy Spirit," Detroit Baptist Seminary Journal (2004), p64–67.
Williams, Renewal Theology, Volume I, p256.
Cited in Aquinas, Summa Theologica.
Chalmers, "On the Nature of the Sin against the Holy Ghost" (alt source, p192).  More from Chalmers on this is available in his Institutes of Theology, Vol. II, Lecture 4 (p472).  
Mueller, Christian Dogmatics, p233.
A number of quotes are given in Schmid, Doctrinal Theology of the Evangelical Lutheran Church, p276–77.
Arminius, Works of James Arminius, Volume 2, p528–29.  For who can commit the sin, see p523–24.  (Alternate source)
Calvin, Institutes, Book 3, Chapter 3, 22.
Calvin, Harmony of the Gospels, Vol. 2.
Berkhof, Systematic Theology, 2.2.4, p252–54.