Question on "Once Saved, Always Saved"

Note:  A friend recently emailed with a question about “once saved, always saved.”  I thought others might profit from reading the interaction (edited):

Q:

Hi Pastor Jeff,

Here's the deal. I was given a statement and was told it is controversial, then it was suggested that I learn about it on my own, then I might ask various men what their thoughts were.  So here's the statement:

Once saved, always saved.

Doing a little research proved to be singularly unenlightening.  Doing more led me to believe that this is a true statement with a caveat or maybe two.  Once saved, I suppose a person could go into some sort of denial, renounce the Lord as their personal savior, harden their heart and thus revert to their prior state: unsaved.  Now then, all that being said I cannot imagine a set of circumstances that would provide someone motivation to do such a terrible thing, but I suppose it could happen.  Most anything's possible, right?  But other than that, I believe that once you are saved, you are always saved.  What do you think?  And which passages should I be concentrating on for this?

JTR:

The term "once saved, always saved" is a more contemporary interpretation of the classic, standard, Reformed concept of "perseverance of the saints" (the P in TULIP).  I personally don't like the "once saved, always saved" expression or the related "eternal security" language, because I believe it tends to water down the fact that genuine salvation is also accompanied by sanctification.

A few years ago John MacArthur got into some back and forth with some "once saved, always saved" or "carnal Christianity" folk over what was called "Lordship salvation."  You should be able to find traces of this discussion on the web.   

Other Resources:

Chapter 17 of the Second London Confession presents a classic Reformed view of the doctrine of the perseverance of the saints.  Can't be said better.

This is my recent sermon on the P in TULIP.

Hope this helps!