If the statement in the title is only what it appears to say, i.e., "once saved always saved," then we could have no argument with it, for according to the Scriptures, once a person is truly saved, then they can never be lost again. However, in many Calvinists minds it can mean the very opposite, so they say you can only be sure that you are saved if you persevere! But first, we need to be sure about eternal security.
The believers eternal security
The simplest and most striking statement about eternal security was made by the Lord Jesus Himself, Who said "My sheep hear My voice, and I know them, and they follow Me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of My hand. My Father, which gave them Me, is greater than all; and no man is able to pluck them out of My Fathers hand" (John 10:27-29). Then the promises of the Bible, in Johns writings for instance, about the connection between a believers faith in the Lord Jesus and the certainty and possession of eternal life, are obviously irreversible. "He that heareth My word, and believeth on Him that sent Me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life" (John 5:24); "He that believeth on Me hath everlasting life" (John 6:47); "He hath promised us ... eternal life" (1John 2:25); and "He that hath the Son hath life; and he that hath not the Son of God hath not life" (1John 5:12). See also John 3:16, 36. If the life spoken about is really everlasting, how can we ever be lost?
Look at the following selected statements made by Paul, Peter, and John about what God has already done for Christians: "sealed us" (2Cor 1:21-22); "quickened us" (Eph 2:5); "translated us into the kingdom" (Col 1:13); "appointed us to obtain salvation" (1Thes 5:9); "called us unto His eternal glory" (1Peter 5:10); "given to us eternal life" (1John 5:11). If it is possible to be lost again after we are saved, then every one of these promises, and many more, would have to be taken away, annulled, or reversed this is never anticipated and clearly impossible!
Gods part in eternal security is further emphasized in such statements as He is "able to keep you without stumbling, and to set you with exultation blameless before His glory" (Jude 24 ESV), and that believers are "kept by the power of God" (1Peter 1:5). Then we read that the Lord Jesus "is able also to save them to the uttermost that come unto God by Him, seeing He ever liveth to make intercession for them" (Heb 7:25), being "the author of eternal salvation unto all them that obey Him" (Heb 5:9). Notice also "having obtained eternal redemption" (Heb 9:12). So "once saved always saved!" Surely we are happy to say with Paul, "I know whom I have believed, and am persuaded that He is able to keep that which I have committed unto Him against [for] that day" (2Tim 1:12).
There are those Calvinists who say that if believers are told that they can never be lost, then this allows them to lead careless lives. (The formal name for such a belief is antinomianism). When we consider the great things that God has done for believers, then "if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. And all things are of God" (2Cor 5:17, 18). Positionally, this is immediately true upon conversion, but as we are obedient to God and His Word, this also becomes true practically. This progress might not be as continual as it should be, so that in the epistles believers have to be warned about stealing, lying, swearing, cheating, being immoral, etc. However, even carnal believers are still sons of God by faith in Jesus Christ (Gal 3:26). The parable of the prodigal son is about a person who was always a son, regardless of what he has done (Luke 15:1124). Even those who have to be excommunicated from assembly fellowship and are under disciple are never said to be lost: they too are still sons.
Many Calvinists, because they are obsessed with the possibility of antinomianism, think that the "once saved always saved" doctrine is actually dangerous. The Charismatic Calvinist R. T. Kendall (author of Once saved, Always Saved, Moody Press, Chicago, 1983, and like Martyn Lloyd-Jones and G. Campbell Morgan minister at Westminster Chapel in London) is reported to have said that "nearly all [17th century Calvinist teachers] went through great doubt and despair on their deathbeds as they realized their lives did not give perfect evidence that they were the elect." Sadly, many Calvinists think the same today. They think that, effectively, at the end of the day, their salvation depends on their faithfulness. However, the Bible says that even, if we are unfaithful, He abides faithful (2Tim 2:13).
The only point left to consider arises from what we said at the beginning of this article: "once a person is truly saved, then they can never be lost again." Sadly we must realize that some people do make false professions of faith because of peer and parental pressure, as well as other reasons, so they were never truly saved at all and often later revoke their profession anyway. (This does not include those who are saved but have doubts about their salvation, for Satan is only too quick to try to rob us of our assurance (cp. Gen 3:1.) Therefore care must always be taken not to illicit false confessions of faith by human means, however well meant.