In his church, he was raised to believe that and he asked "So you're saved even if you later on renounce your faith?" The answer he got was that if that happened, you were never really saved in the first place.
That answer seems like kind of a cop out to me.
That's because it is a cop out. It's the popular no-true-Scotsman fallacy, or in this case, the no-true-exchristian fallacy. They believe that once someone genuinely accepts Christ as their Savior, they can never disbelieve or walk away from the faith.
Now people who don't believe in OSAS have no problem acknowledging that exchristians exist. To them, these are people who have accepted Christ at one point in their life, but for various reasons, have walked away from their faith and have lost their salvation as a result of that.
For the believer of OSAS, there is a problem here, since salvation cant be lost. When someone walks away from their faith, sometimes even openly rejecting the existence of the God they once believed in wholeheartedly, is it logical to believe that God still keeps them saved? Most people will say no, and so they have no choice but to believe that the person must have never been genuinely saved to begin with. They are backed into a corner here with this belief. It doesn't matter how strong the person's testimony is. It doesn't matter how much evidence they give as to how much they truly believed in and loved God at one point in time. It really doesn't matter what sort of evidence is presented or shown, the believer of OSAS is forced to believe that the person must have never been a true believer solely because they are now no longer a believer. Herein lies the fallacy that they must accept in order to keep their beliefs rational.
Of course, there are some progressive OSAS believers who acknowledge that people can fall away, but they would still be saved. They draw their defense mostly from the argument of time and how God exists outside of time. In today's world, there is so much emphasis placed on where you are spiritually at the time of your death. If someone was a genuine believer since age 15, but no longer believed for their last 5 years of life, what (other than time) makes this person different from someone who was a nonbeliever all their life but accepted Christ and lived 5 more years before they died. Both of them had a period in their life where they truly and genuinely believed in and loved God and accepted Christ as their savior. Both also had periods where they did not believe. Yet...due to timing alone...one goes to heaven and the other to hell. That's why some argue that once you are saved, you are always saved no matter what (just don't expect many rewards in heaven in some cases).
Either way, the important thing is to make sure you aren't backing yourself up into a theological corner. Former believes exist out there. I know way too many Christians believe they don't exist, but they do. So when it comes to OSAS, you need to acknowledge that and decide whether you believe in OSAS acknowledging that these people who walk away are still saved or if you don't believe in OSAS and these people are now unsaved. I would say search the Scriptures, but I'm sure you know that the Bible seems to have a litany of verses supporting both sides of the argument. In this Biblical gray area, one must rely, at least in part, on logical reasoning.