Jason Stellman (former Calvinist, now Catholic) made a brilliant observation on his blog a while back about the 'unpardonable sin' that Jesus speaks of in the Gospels. The verse states,
And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. (Mat 12:31-32; Mk 3:28-29; Lk 12:9-10)
Catholics have traditionally appealed to the phraseology of forgiveness "in the age to come" as evidence for purgatory, since this is the only reading that makes sense of the idea of sins being able to be forgiven after death. Some Protestants have tried to dodge this by appealing to the parallel accounts where Jesus says "will never be forgiven," but I call this a dodge because it doesn't get around the wording of "or in the age to come," it merely affirms what's already agreed upon: the sin is never forgiven.
But that's not what Jason pointed out that I thought was worth sharing. What Jason pointed out was that this verse goes against the Protestant idea that salvation cannot be lost (known as Eternal Security or Once Saved, Always Saved). This is because the text says that Jesus is speaking of all kinds of sins being able to be forgiven, just not this one. This suggests a person can go 'too far' with their sins and thus cross an 'invisible line' of no return. They can sin one too many times or they can sin in such a severe way that they wont be forgiven and thus they will be damned.
The only counter I can envision a Calvinist making is that this 'unpardonable sin' is simply talking about those who were never saved to begin with. But this makes the unpardonable sin synonymous with being in an unconverted/unregenerate state (i.e. born in original sin), which makes no sense for two reasons. First, everyone is born in the unconverted state, so this cannot be the unpardonable sin. Second, the context Jesus is speaking is that of sins that can be forgiven, just not this one, indicating it's a sin yet to be committed by any particular person.
All believe that there is no sin for which the Cross could not atone for, which means this 'unpardonable sin' has to be something more pernicious than sinful acts in general. The traditional Catholic reading of this text gives two interpretations, which overlap somewhat. The first is discerned by the context, where the Pharisees had seen all these miraculous signs coming from Jesus and yet they continued to refuse to believe in Him. What made this moment especially wicked was that they attributed the miracles to Satan rather than the Holy Spirit! This repeated willful blindness caused them to be permanently blind to seeing the light and thus beyond able to repent. They reached a point where God stopped trying to convert them. The second and more generic interpretation is that the 'unpardonable sin' is the sin of 'final unrepentance', meaning willfully refusing to repent of being in a state of mortal sin up to the moment of death. So someone who is out of communion with God who refuses to repent and dies in that way has put them self in a state that precludes forgiveness. Either way, Eternal Security is refuted by this verse.