Wow that is a deep question brother Todd:-) If you had asked me this question 6 years ago I would have given you a cookie cutter answer inline with my denominational beliefs. But since then I see things a lot differently. I now see salvation in much the way Paris Reidhead describes it in his multi-part sermon series "So great a salvation".
Salvation is not a cut and dried thing, there are no guarantees. We never actually have it "in the bag" for if that were the case we would not have this "blessed hope". The calvinists see it differently to me in that they believe that people are hand selected to be saved and no matter what, this elect group will be saved regardless.
I do believe that salvation is a process and that some christians - having first begun in faith will for a variety of reasons fall away from the Lord and be eternally lost.
The Lord will never write them off though and will continue to lovingly urge them and warn them to repent and return to Him. The warnings are just like those in Revelations 2 & 3. For example to the church at Sardis the Lord warns them that if they do not wash their filthy garments they will have their names erased from the Book of Life. So it is the same for us, there will be a sifting and a shaking going on in our lives as the Lord continues help us follow after Him.
At the same time as we approach the end of this age we will see a great (greater) falling away that is characterized by a false church system (babylon) and a deathly religious spirit that has the perfect form of what a christian should look like but is in fact empty and void of Christ. This is apostasy. Now, true believers who remain in contact with this "form" and continue to be influenced by this apostate church will be in danger of falling away themselves. And this is why I believe the Lord is sifting and shaking believers to see the truth and to understand that not all is as it seems. That the "emperor" is not wearing any clothes.
I believe that the more the Lord sifts and shakes the more whole hearted believers will see the false church in more and more of its forms and gradually remove themselves from more and more of it.
But it is a process. At first the shaking creates an awareness, an awakening to the obvious things like worldliness or the love of money. And so believers remove themselves from the groups who fool around in these areas. And then the Lord shows whole hearted believers more and they see that many of religious groups are devoid of the life of the Spirit...they are not just in need of revival...they are "Ichabod". And so the whole hearted believer comes out of those groups as well.
And so on and so on.
This is what I mean by a sifting and a shaking...sorry if this sounds disjointed and muddled I wrote it in a hurry.