Like all things Protestant, Calvinism is divided, even on its core concept of the “perseverance of the saints.”
On one side are those Calvinists who believe that all true believers will persevere until the end and be saved, because God will make it so. In other words, once saved, the true believer cannot help but to persevere, never committing apostasy or falling from the state of grace. This is the traditional Calvinist view found in the Westminster Confession of Faith.
The other camp is sometimes referred to as the “free-grace” Calvinists, and its position is that, after “being saved,” no action, no matter how grave or frequent, can change one's status with God. True believers are not, as in the traditional view, divinely protected from “falling away,” but falling away has no eternal consequences.
Despite their differences, though, these ideas do share common ground: the denial of mortal sin.
The sin problem
In Catholic teaching, mortal sin is a grave sin committed with full knowledge and consent that “turns man away from God” (CCC 1855). In traditional Calvinism, however, no true believer ever commits a grave sin with full knowledge and consent, because to do so would demonstrate that he was not a true believer after all. If he wasn't a true believer, then he was never in a state of grace to begin with, making mortal sin impossible. (“State of grace” is the actual language of the Westminster Confession.)
Free-grace Calvinists take it a step further, holding that nothing a true believer does, no matter how sinister or grave, will ever change his standing before God. Even a person who experiences a conversion of repentance and faith in Jesus Christ and later abandons that faith, committing apostasy and persecuting the Church—imagine if St. Paul returned to making martyrs—that person is ever and always in God’s grace and destined for eternal joy in heaven.
In this world of complete eternal security, sin doesn’t exist. This sort of eternal security requires the denial of the very possibility of sin, either for the “elect" or for the “reprobate.”
What sin really is
Often we make the mistake of thinking of sin as simply something God hates, and, accordingly, think of the consequences of sin as God’s response to it. That’s not entirely accurate. Sin is, by nature, that which separates us from God. The word comes from the Greek sunde, which we recognize in the word sunder. To sin is to separate. If an action moves us away from God, that’s why it’s sin. God didn’t choose that any number of things would be sinful and then impose the penalty of separation upon those who did them.
In free-grace Calvinism, though, the believer never suffers the separation that is inherent to sin, and thus it denies the nature of sin. For traditional Calvinism, because the nonbeliever is hell-bound and “totally depraved” anyway, no action has the potential to increase or diminish his participation in the grace of God. For this "reprobate," sin and good deeds are both impossible. The nature of sin is denied again.
As a Protestant, I recognized this error and couldn’t pretend it didn’t exist. Traditional Calvinism may differ from the newer free-grace variety, but neither one has a leg to stand on. You cannot deny the nature of sin without denying the nature of God, for God is goodness, and sin is the privation of goodness in action. If God is good, then sin is always sin, and “perseverance” is nonsense.