Unless otherwise indicated, all Scripture quotations are taken from the New American Standard Bible (1995 edition).
When I was in middle school, I discovered Internet message boards. My parents had purchased one of the America Online early versions (when dial-up was the only thing available), and I discovered a whole new world of discussion boards where people would post what they thought online. And they would often disagree. That was when I first started dialoguing with non-Christians—people who didn't already believe the same things I did.
I was one of those naïve evangelical kids who had been taught that anyone who claimed to be a "former Christian" was never really a Christian to begin with. That is, they never truly understood that one had to be saved by grace, not by works.
So to show off my obviously "Christian-exclusive" knowledge and maybe save some souls in the process, I got into a conversation with a self-proclaimed "ex-Christian" on a discussion board that concerned the topic of religion. She said she once believed in Jesus but could no longer do so. Having been taught that such people weren't true Christians to start with, I protested and arrogantly posted something in response like, "Nah. You were never a true Christian. Look, I dare you to try and describe what Christians believe about how we get saved. Go ahead. I'd be surprised if your answer is correct."
When I posted the comment, I leaned back in satisfaction. She's probably going to say that Christians are saved by the works of the Law or something. She couldn't possibly understand that true Christians believe we're saved by grace. So when she gives me the wrong answer, I'll explain to her what Christianity is really all about!
A couple hours later, I checked back for her response. She had written, "According to Protestant Christianity, a person is saved by grace through faith. Jesus supposedly died on the cross for our sins, and it is only by believing in Jesus that one can be saved from the fiery pits of hell. Yeah, right. I'm so glad I don't believe that anymore."
I didn't respond. But I knew she had nailed me. I suppose I could have written back, "Ha! I knew you were going to say that! But did you really believe it?" Of course, then I would have made an even bigger idiot out of myself.
Think about this: Does it help for us to say, "You were never really true Christians to begin with" to people who say they used to be Christians? Why do we say this? Is it because that's what we think we're "supposed" to say? Do we have any good reasons to say it?
Some relevant verses to consider:
"My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand. My Father, who had given them to Me, is greater than all; and no one is able to snatch them out of the Father's hand" (John 10:27-29).
"They went out from us [Christians], but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us" (1 John 2:19).