Once saved always saved. Is eternal security Biblical?
Recently, I had a conversation with a (new) friend that started off with him telling me how he had been a devout Christian, perhaps even a believer (to use some Christianese), but had lost his faith due to later events. This struck me as a bit odd, because I had always thought that once you had put your faith in Christ, then you could never lose it. I went hunting for the verse that I was thinking of (and that he confirmed as existing), but couldn't find it. So...was I right? Is it true that once you put your faith in God and Jesus, then you can never lose it, especially under persecution?
To clarify, I'm more interested in whether someone else can cause you to lose your faith as opposed to you turning away from God. The "Once saved always saved." question deals with people turning away from God, but does not deal with other people "causing" a deconversion.
Biblically-based answers are prefered.