The Bible is Clear: ‘Eternal Security’ is a Manmade Doctrine
The notion of ‘once saved, always saved’ is false and presumptuous. Mortal sin is possible — even for believers.
Eternal security (or, what is called perseverance of the saints in Calvinism) is the Protestant notion that, once having attained salvation, a Christian can never possibly lose it, no matter what they do. This is a false and unbiblical doctrine.
Catholics, Orthodox and many strains of Protestants, on the other hand, believe that it’s possible for a regenerate, justified Christian to reject God and his teachings and fall away (apostasy).
Catholics also believe in a strong notion of “moral assurance” of salvation, meaning that if we examine ourselves and make sure we are free of all mortal sin, that we are in good graces with God and would be saved and go to heaven if we died at that moment.
In this article (and continuing in the next installment), I present much biblical data that refutes eternal security and upholds Catholic teaching on justification and salvation.
Romans 11:22 (RSV) Note then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness; otherwise you too will be cut off.
St. Paul is talking primarily about groups (Jews and Gentiles) and “communal salvation,” so to speak, so this is a relatively weak argument against the eternal security of individuals. That said, it’s still, nonetheless, the same general notion that groups (like individuals) can possess salvation and then lose it (“those who have fallen” / “cut off”).
It’s not instant (as shown by the words, “provided you continue...”). The passage is also reminiscent of Jesus’ sayings about individuals who will be “cut down” and damned if they fail to produce “fruit” (Mt 3:10; 7:19).
1 Corinthians 9:24-27 Do you not know that in a race all the runners compete, but only one receives the prize? So run that you may obtain it.  Every athlete exercises self-control in all things. They do it to receive a perishable wreath, but we an imperishable.  Well, I do not run aimlessly, I do not box as one beating the air;  but I pommel my body and subdue it, lest after preaching to others I myself should be disqualified.
St. Paul refers to an “imperishable” wreath. That’s eternal life, that never ends, or at the very least, eternal rewards that come as a result of having been saved. Being “disqualified” is a rather obvious reference to possible loss of salvation, if we don’t persevere.
Salvation is also evident in context, with references to winning men (i.e., playing an instrumental role in helping them to become saved: five times in 9:19-22). He refers to preaching the “gospel” (9:14, 16, 18, 23). That has to do with salvation. And he writes, “that I might by all means save some” (9:22). It’s all salvation. And it can be lost: so teaches the Apostle Paul: the greatest evangelist of all time.
Galatians 5:19-21 Now the works of the flesh are plain: fornication, impurity, licentiousness,  idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit,  envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God.
Why warn them about the possibility of not “inherit[ing] the kingdom of God” if they are in no danger whatsoever of losing it? That makes no sense. Why would Paul use the word “warn” if it didn’t also apply to real potential danger in the spiritual lives of his Galatian recipients? The early part of the chapter makes it crystal clear that a Christian can fall away from the faith:
Galatians 5:1, 4 For freedom Christ has set us free; stand fast therefore, and do not submit again to a yoke of slavery. . . .  You are severed from Christ, you who would be justified by the law; you have fallen away from grace.
Ephesians 5:3, 5 But fornication and all impurity or covetousness must not even be named among you, as is fitting among saints. . . .  Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God.
Paul is here teaching that habitual / “lifestyle” fornication and other such serious, mortal sins can lead one to hell, and he warns the Ephesian Christians not to fall into the sin. If they do — the implication is clear — they put themselves in danger of damnation: in just as much danger as the ones who never were Christians; maybe more so, on the biblical principle of “Every one to whom much is given, of him will much be required” (Lk 12:48).
Hebrews 10:26-29 For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins,  but a fearful prospect of judgment, and a fury of fire which will consume the adversaries.  A man who has violated the law of Moses dies without mercy at the testimony of two or three witnesses.  How much worse punishment do you think will be deserved by the man who has spurned the Son of God, and profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace?
A “full assurance of faith” is referred to in the immediate context (10:22), but then in the next verse we are told that we must “hold fast the confession of our hope without wavering.” Again, if wavering or falling is impossible from the outset and poses no danger, then why is it mentioned at all?
The same indications of possible falling away occur after our passage above: “do not throw away your confidence” (10:35), “you have need of endurance” (10:36), “those who shrink back and are destroyed” (10:39). One doesn’t refer to a group of people who commit terrible sins and lose the faith, if indeed it’s not possible in the first place.