“Whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”
In my opinion, the best Protestant critique of Catholicism in our times is the book, Roman Catholics and Evangelicals: Agreements and Differences, by Norman L. Geisler and Ralph E. Mackenzie (Grand Rapids, Michigan: Baker Books, 1995).
Dr. Geisler (a very notable, credentialed evangelical apologist and professor of theology) lists Matthew 12:32 as one of four primary biblical proofs that Catholics offer for purgatory (p. 333). Here it is:
Matthew 12:31-32 (RSV) Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven.  And whoever says 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.
Catholics argue that this passage makes reference to forgiveness after death: something that is anathema to Protestantism. This particular super-serious sin (blasphemy against the Holy Spirit) is not forgiven, yet it is clear that Jesus is presupposing that there are other sins that are forgiven after death: which is one of the tenets of purgatory: forgiveness for and temporal punishment of sins after death for the person who is already saved and will inevitably make it to heaven in due course.
The passage was interpreted in this fashion by several great historical exegetes; for example, St. Augustine (354-430):
For some of the dead, indeed, the prayer of the Church or of pious individuals is heard; but it is for those who, having been regenerated in Christ, did not spend their life so wickedly that they can be judged unworthy of such compassion, nor so well that they can be considered to have no need of it. As also, after the resurrection, there will be some of the dead to whom, after they have endured the pains proper to the spirits of the dead, mercy shall be accorded, and acquittal from the punishment of the eternal fire. For were there not some whose sins, though not remitted in this life, shall be remitted in that which is to come, it could not be truly said, “They shall not be forgiven, neither in this world, neither in that which is to come.” (City of God, Book 21, chapter 24).
Others who interpreted it similarly include Pope St. Gregory the Great (c. 540-604): Dialogues, Book IV, ch. 39; St. Bernard of Clairvaux (1090-1153): Sermon 66 on the Song of Songs; and The Venerable Bede (c. 672-735): Commentary on Mark 3.
Geisler contends (p. 335):
How can the denial that this sin will . . . ever be forgiven, even after death, be the basis for speculating that sins will be forgiven in the next life?
I reply that mentioning “the age to come” assumes the premise that such things can happen in the afterlife; after death. Otherwise, why mention it? We don’t include in our observations what we regard as a falsehood or impossible. No one would say, for example, “a circle is round and also is square.” The first thing is obviously true, and the second is categorically impossible.
If Jesus thought (like Protestants) that there is no forgiveness (or purgatory) after death: as a categorical impossibility, then I submit that He would have never mentioned even its theoretical potentiality. He simply wouldn’t bring it up at all. He doesn’t teach falsehood, being God and omniscient.
The “polemical structure” of this Catholic argument is similar to the biblical argument for praying to someone other than God, found in Luke 16:
Luke 16:24 (RSV) “And he called out, ‘Father Abraham, have mercy upon me, and send Laz’arus to dip the end of his finger in water and cool my tongue; for I am in anguish in this flame.’”
Abraham says no (16:25-26), just as God will say no to a prayer not according to His will. He asks him again, begging (16:27-28). Abraham refuses again, saying (16:29): “They have Moses and the prophets; let them hear them.’” He asks a third time (16:30), and Abraham refuses again, reiterating the reason why (16:31).
If in fact we are never supposed to ask saints to pray for us, I think this story would be almost the very last way to establish that point. Abraham would simply have said, “you shouldn’t be asking me for anything; ask God!” In the same way, analogously, angels refuse worship when it is offered, because only God can be worshiped (Rev 19:9-10; 22:89). St. Peter also refused worship (Acts 10:25-26), and so did St. Paul and Barnabas (Acts 14:11-15).
Likewise, if there is no such thing as forgiveness “in the age to come” Jesus would not have alluded to it. The fact that one sin can't be forgiven even in the next life does not prove that none can be forgiven, just as Abraham's (not God's!) refusal to grant one prayer does not prove that no one can pray to anyone other than God.
Geisler then argues (p. 335):
[T]he passage is not even speaking about punishment, which Catholics argue will occur in purgatory. So how could this text be used to support the concept of purgatorial punishment?
But this is not the Catholic argument in the first place; it is, rather, that forgiveness of sins after death is one essential aspect of purgatory, which this passage supports. It’s a partial proof of purgatory, not a full one (every particular aspect of purgatory).
He continues (p. 335) to miss the main point:
[P]urgatory involves only venial sins, but this sin . . . is mortal, being eternal and unforgivable . . . even if this passage did imply punishment, it is not for those who will eventually be saved . . . but for those who never will be saved . . . It only indicates the lack of real biblical support for the doctrine.
Remarkably enough (for such a generally good debater), he never engages the argument (explained above) as actually put forth by Catholics.