One of the most humorous (and maddening) doctrines to discuss with a Calvinist is how Calvinism deals with the subject of apostasy (i.e. falling away from the Christian faith). Since Calvinism teaches that the 'true believer' can never lose his salvation, this naturally leads one to ask how Calvinism deals with the "warning passages" in Scripture. The "warning passages" are all those passages which warn about the danger against turning to sin, particularly grave sins which can cause one to be damned. An excellent example of this is Galatians 5:19-21, where Paul (for the second time) warns the Galatian Christians that if they commit grave sins they will be in jeopardy of not entering the kingdom of Heaven.
Calvinists approach the "warning passages" with a sort of double standard. On the one hand they say that anyone who commits those sins was probably "never saved in the first place," while on the other hand they admit a 'true Christian' could fall into those sins but that God has pre-forgiven all their sins since the moment of their conversion and justification. (I discuss this inherent-contradiction in my Lordship Salvation post.) But there is yet another damning contradiction to go along with this, and this stems from the fact Protestants in general (and Calvinists in particular) reject the Catholic distinction between mortal and venial sin.
The problem the Calvinist is in is simply this: if there is no distinction between mortal and venial sin, then all sin is equally grave and thus equally damning. And if even Christians sin in "small" things many times each day (Prov 24:16), this leads to the terrifying realization that they're committing damnable sins throughout each day. This error and failure to follow the Church caused Luther to be deeply distressed, and logically so, which in turn was passed onto Calvin and eventually most all Protestants. This forced Luther and Calvin into having to invent the doctrine of the "Imputation of Christ's Righteousness," where Christ's Righteousness would "cover" the believer and effectively hide their daily repeated (mortal) sins from God's sight. Protestants call this God "not imputing" sin, meaning God knows you commit all these grave sins each day, but since you're "covered" by Jesus' righteousness then God will graciously not count you guilty for them. But this only compounds the problem at hand rather than alleviate it.
Since the Calvinist Christian is committing the very damnable sins warned against in places like Galatians 5:19-21, 1 Corinthians 6:9-10, Ephesians 5:3-5, and 1 Thessalonians 4:3-6, the Calvinist really cannot explain how these are "warnings" at all if they're virtually inescapable even by Christians. This all but makes these "warnings" complete jokes and naturally should lead one to reject the Calvinist view in virtue of the fact Calvinism reduces to absurdity on this point. The only way to explain these texts is to recognize the mortal and venial sin distinction, which is why these texts are clearly singling out certain grave sins and not speaking of every sin being damnable. But that would require Protestants to reject Sola Fide, which isn't going to be easy for them to do.