Question: Can you reconcile the Baptist credo, "Once saved always saved" with II Peter, 2/20 which seems to imply that you can lose salvation by lapsing back into sin, "...[having] the knowledge of the Lord ...they are again entangled...and the end is worse..."?
Response: Peter's Epistles #27 (currently the last in that series) deals with the false doctrine of "eternal security" in detail. We can debate until the cows come home whether Calvin really believed "once saved always saved". My take on the original reformed tradition is that Calvin and co. believed that if you fell away from the faith, became apostate, etc. you never were really saved in the first place. As I lay out in detail in Pet.#27, I believe the Bible to be exceptionally clear on the point of the necessity to maintain faith for salvation. This makes good sense, for if we are truly no longer believing in Christ, in what way are we Christians? The foolish virgins' lamps are the lamps of faith that expire under the pressure of the Great Tribulation and they are shut out of the feast, and told by our Lord "I never knew you". Knowing what most Baptist commentators and theologians mean by "once saved always saved" I would say that this is a might hard to square with many passages in the Bible, including your example of 2Pet.2:20, and you very correctly see that sin is the issue. To put the matter succinctly, sin and following Christ are antithetical; all of us sin, but there is a difference between stumbling and repenting on the one hand, and giving oneself over to a life of sin on the other. As Christians, we know sin is wrong; if we embrace it, we will come to "hate the light" and eventually turn away from the light entirely. Now I am sure that many people in the history of the Church have been able to "crawl sideways", never getting in deep enough before repenting and turning back so as to lose faith entirely; but that is not a profitable way to live the Christian life. Naturally, for all those who are earnestly striving to please our Lord and Master, the doctrine may seem academic (i.e., they wouldn't surrender to sin so as to lose faith even if they believed "once saved always saved"). But we live in "interesting times" (as the Chinese curse goes), and if it be the case that it is this coming generation that will have to pass through the cleansing fire of Tribulation, then the mistaken belief that NOTHING can destroy faith (and so threaten salvation) will be a dangerous one indeed.
Please see also the following links:
The False Doctrine of Absolute Eternal Security.
The False Doctrine of Absolute Eternal Security II.
The Great Apostasy
Apostasy and the Sin unto Death
Thanks for your interest. Hope this helps in some way.
Yours in Christ,