You know OSAS, right? "Once saved, always saved." When I was growing up, I thought I was a Calvinist because I believed "once saved, always saved" and those miscreant Arminians didn't. Of course, I didn't realize that "Calvinism" (poorly named) has 5 points and I was ignoring 4 of them. Imagine my surprise, then, when I came across the rest of them, saw the Scripture involved, and rejected OSAS.
"Okay, now, hold on," I can hear you saying. "Aren't you a Calvinist? Don't you subscribe to that point as a Calvinist?" No, I don't, and I'll tell you why. I cannot tell you how many people in my lifetime I've heard tell me "I'm a Christian" while they indulged in open, admitted sinful practices which they defended on the basis of "OSAS". "You can't tell me I'm going to hell," they would tell me, "because I accepted Christ when I was younger and once saved, always saved, right?" This is a problem.
"Why?" some might ask. "Because it violates your views?" No, because it violates Scripture. It ignores all the biblical warnings about testing yourself and falling short. It ignores all the biblical statements about the nature of a genuine believer. John wrote something that was startling. "No one born of God makes a practice of sinning, for God's seed abides in him, and he cannot keep on sinning because he has been born of God." (1 John 3:9) When Paul wrote, "If anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come" (2 Cor 5:17), it wasn't trivial. OSAS would argue that it is not only trivial; it is wrong.
If I told you, "It is God who works in you, both to will and to work for His good pleasure" (Phil 2:13) and left it at that, I think you'd find that I had told a half-truth. That is, left alone, it sounds like God working in believers is all that is needed and we can just go about our merry way. But the statement is actually at the end of a previous sentence. "Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for ..." (Phil 2:12-13) Yes, it is God who works in you, but this is something He does so you will do something. So you will do what? So you will "work out your own salvation." "Well, sure," you might say, "but we're not saved by works." No, we aren't. But this working out of salvation is not minor or incidental. It is accompanied by "fear and trembling". Yes, God is at work in us to have both the will and the power to do it, but we must do it. Where does that leave the OSAS side which insists we don't have to do anything? Where is the fear and trembling of adherents to OSAS? Going with the popular perspective of OSAS, where we can sin all we want once we're saved because salvation can't be lost, it would suggest that we don't have the sense that demons have (James 2:19).
The other problem is that OSAS is cavalier about salvation. It is more of a "I've got my 'get out of hell free' card so I can do whatever I want" mindset. It doesn't take into account the cost. It doesn't take into account the change in nature that a relationship with Christ brings.
I should be clear here. I'm not saying you can lose your salvation. If you are able to simply go on sinning to your heart's content, you never had it. That's what John said. "They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us." (1 John 2:19) Jesus said that those who are in the Father's hand cannot be removed (John 10:28-29). Paul said, "He who began a good work in you will bring it to completion at the day of Jesus Christ." (Phil 1:6) If He does not, the work was not begun. The work that we do doesn't secure our salvation, make us more acceptable to God, or anything of the sort. It is Christ working in us. From that perspective, we cannot not work. The "5th point" is not "once saved, always saved". It is the claim that God will keep His own and that, through the working out of His power in our lives, we will be changed, transformed, sanctified, conformed to the image of Christ. No change? No salvation to start with. Saved? Then change will occur. It's as simple as that.