In Calvinism, the fundamental thing that separates the saved from the damned is faith. Those to whom God wants to save will be given the gift of faith, while those whom God does not want to save will never be given the gift of faith. In other words, in the Calvinist world view, there's nothing worse than being an unbeliever. But if this is the case, why does Saint Paul say it's possible to be "worse than an unbeliever"?
I believe 1 Timothy 5:8 is the scariest passage in all of Scripture, for it gives everyone a lot to think about: "If anyone does not provide for his relatives, and especially for members of his household, he has denied the faith and is worse than an unbeliever." St Paul is talking to Christian parents here, who have a grave obligation to care for their family. For those Christians willfully failing this duty, Paul has a most stern rebuke: they have denied the faith and are worse than an unbeliever. So contrary to Calvinism, what is worse than an unbeliever is a Christian who falls away from the faith.
Anyone who's an adult should have some fear in reading this passage, for they must realize that this could happen to them. But Calvinism expressly denies such a thing is possible. As such, they must try desperately to spin this verse to mean either the person in question was either (a) never really saved, or (b) that a Christian cannot really commit this sin.
The problem with saying they were never really saved is that such an assertion is purely begging the question and even goes against the plain reading of the text. To "deny the faith" in this context is clearly an act of apostasy. While Calvinists believe in apostasy, they just believe anyone who falls into apostasy was never saved, which makes no sense. One cannot fall away if they were never a member to begin with. The language of "worse than" an unbeliever further testifies to the fact this person is not an unbeliever, but rather of another category, that of being saved and not persevering (2 Peter 2:20-21).
The problem with saying that a Christian cannot really commit this would likewise be begging the question, and even presuming Paul wasn't serious. But even if Paul didn't think a Christian could commit this sin, Paul would still be making an inaccurate theological claim by saying there is such a thing as "worse than an unbeliever."