"But the one who endures to the end, he will be saved" (Matt 24:13)
True saving faith is one that endures. Saving faith has been granted freely by God, not based on any decision of the human will (John 1:12-13, Romans 9:16). Contrary to misconception, this is a genuine faith, it is an active and vibrant belief in Jesus Christ. God isn't believing for us. Some like to caricature the Perseverance of the Saints by calling it "Once saved always saved." While that phrase is true, what scripture says is much more poignant than that. What Christ says in John 6:39 is far more substantive, "This is the will of Him who sent Me, that of all that He has given Me I lose nothing, but raise it up on the last day." Essentially, what Jesus is saying is that He will not fail in bringing His people to salvation. Jesus describes those with true faith in the parable of the soils as "...the one on whom seed was sown on the good soil...."(Matt 13:23) The ones with false faith are described as having "...no root...," or "sown among thorns." (Matt13:21, 22) One that has made a profession of faith and has since gone back to the world, never had the type of true faith that Jesus was talking about. "They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us." 1 John 2:19 is clear that those who depart from the body never really were a part of it.
The Calvinist can hold to this doctrine because it is based on God's gracious efficacious call of His elect, the irreversible work that God does in the fallen sinners heart. God's work cannot be undone. It is based on the fact that none can be snatched out of Jesus' hand (John 10:29). Our salvation is based on God's firm grip on His children and not human striving. But what about the Non-Calvinist? What about those who say that God leaves the ultimate decision in salvation up to the rebel sinner? Can they hold to this doctrine and remain consistent?
My Non-Calvinist friends sometimes say that man makes the final decision in salvation, then God keeps them from falling away. But there is an implicit assumption in the first part of that proposition, the assumption being that we are free to make that decision. For the sake of illustration let us say that we are free to make that eternal decision. What happens to that freedom after the choice has been made? If it depends on the person to enter into salvation, isn't it up to the person to keep himself saved? If God keeps the person from falling away after they have cast the deciding vote, then does it not follow that God has now taken his freedom to fall away? This illustration shows the inconsistency of free-willism, or any theology that gives man an ability that the Bible does not. One cannot hold to the Perseverance of the Saints when the other points of Calvinism are denied, especially: Unconditional Election, Limited Atonement, and Irresistible Grace. For these are the very basis by which God draws His people unto himself and into eternal salvation. In Calvinism the elect are brought into salvation infallibly without any reference to what they will do. Since God, not man, elects and draws, then it follows that He preserves the saints which causes them to persevere. Salvation is wholly the work of God, there is no concept of synergism in the Bible.
"And then I will declare to them, 'I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.'" (Matt 7:23)
Please take notice that Jesus does not say "I knew you once, but you have fallen away!" God knows His people from all eternity because He is the one who initiates and completes that relationship, not man (Romans 8:29, Ephesians 1:4). One cannot hold to the Perseverance of the Saints when God is denied credit for the whole of salvation. This doctrine cannot be held when the very way in which God draws His people is rejected. In my Christian life I have found myself asking the question "why me?" Why do I believe and others do not? Why do I love Christ while others continue in their hatred for Him? Surely it isn't me who is better than anyone else! But that's exactly what the Non-Calvinist is saying when they claim that man is free, because they place the deciding factor of salvation in man and not God. When they assert man's freedom, they deny Unconditional Election, because they make man's choice the condition that moves God's hand. Limited Atonement is the logical consequence of Unconditional Election, the saints are elected unto the atonement. Irresistible Grace is the work that applies the atonement to the elect. The Perseverance of the Saints follows because what God has done cannot be undone. He elected, atoned for the elect, and draws the elect unto Himself. Of course He will ensure that they will persevere. This holds together in Biblical Reformed theology. So on what basis can the Non-Calvinist hold to the Perseverance of the Saints? There is no basis other than Unconditional Election, Limited Atonement and Irresistible Grace. When the Non-Calvinist denies those, they hold to the Perseverance of the Saints without foundation.