The question of whether or not people can “lose” their salvation, to the extent that this language even makes sense, has been traditionally controversial. In the time in between the completion of the New Testament and, say, St. Augustine, competing views on how salvation works, who will enjoy it, and how we truly receive it proliferated. Augustine wrote of many positions he had heard of, everything from “only persevering, faithful, orthodox, baptized Christians will be saved” to “everyone will be eventually saved.” It would be hard to pick out one as the most common for a long time.
Augustine himself is notable for his belief that, while only certain Christians were predestined to persevere and finally be saved, other Christians could still be Christians but not persevere and so not be saved. This position seems to have set the basic tone for the Catholic Church for the next 1000 years or so. With the Reformation, views began to multiply yet again, with most of the Calvinist/Reformed holding to perseverance of the saints (specifically, the true Christians, who are God’s elect, will persevere in the faith by the work of the Spirit, and they will be saved), Lutherans coming to believe that salvation, given at baptism, could be lost through unbelief, and Arminians believing pretty much either way (though eventually the position that you can lose salvation became the standard for them).
Today, views are nearly as diverse as the early church, though in evangelical Protestantism a few of the early views (like universalism and baptism as absolutely required) are mostly absent. In the really basic, everyday evangelical/Baptist/Pentecostal/nondenominational world I’ve always lived in, you can identify two basic, common views. They are:
(1) that once you truly believe in Jesus with authentic, saving faith, you are presently saved and assured final salvation with no possibility of loss, and the Holy Spirit will keep you from falling away permanently, and
(2) that once you truly believe in Jesus with authentic, saving faith, you are presently saved but only assured final salvation inasmuch as you continue to trust in Christ, which you might cease doing if you choose.
Both of these have their own ways of interpreting the Biblical evidence, but obviously both cannot be true. Either one is right, the other is right, or, perhaps, both are wrong and another conception of how salvation works might be true (e.g. some people believe that any belief in Jesus, even obviously dead faith which immediately changes its mind, guarantees final salvation, and some people would require a host of other things).
I write because I am going to write two essays, one representing each side of this debate, from a very specific vantage point. Given that Jesus is the center and source of our salvation, and our connection to Him by the Holy Spirit is essential to the whole question, I think it makes sense to approach this issue from the angle of union with Christ. Salvation consists of us being “in Christ,” to borrow a phrase from Paul (Rom. 8:1, 1 Cor. 15:18, 2 Cor. 5:19, Gal. 3:28, Eph. 1:3, 4:23, Phil. 4:19, Col. 3:11, 1 Thess. 5:18, etc.). As such, I want to present unbiased arguments for both views using union with Christ as the controlling concept. Hopefully, this will be helpful and enlightening, and perhaps help each side converse more clearly and charitably.