Aww, it's alright! I didn't take any of it personally, or even see anything in your posts that require an apology, really. But it's very kind of you to do so.
By the way, the scripture you said you would like -- the one that implies that those who leave the faith never had it in the first place -- is 1 John: 2:19. To answer your question of "how do I know", my answer is: 1. that scripture I just referenced. And 2. Different testimonies from various people who were in that situation.
As for your personal story, the reason you came back could have been because God knew what circumstances, conversations, etc. to bring your way that would show you the way back. God doesn't force anyone, but he does know how to prod people. I doubt he would have let you die before you came back to Him. Going back to the scripture that says He lets noting snatch us out of His hand, I believe that also applies to the doubt that comes to us when childhood faith leaves, and the doubt of our teen/adult mindset sets in. Especially considering that it's something that happens to almost every Christian. (I haven't known many, if any, who haven't gone through it to some extent.) But, of course, this is all just my guess based on my beliefs. Plus, I didn't see your heart during the time you were little, or during that time of doubt, so really only God knows for sure what did/would have hapenned.
Like you say, a lot of these scriptures can mean different things according to interpretation, which is what makes it so impossible to win the argument, no matter what side your on. Eternal Security vs. Non-Eternal Security is a debate that famous theologists have been going through for years, and probably will be continuing for years to come. So, I doubt it will be finally resolved anytime soon -- especially by us two non-theologists, lol.
But the conversation was fun while it lasted! Thanks for talking with me about it Lirenel! (And Gilby's Angel)

~Riella

Thank you for accepting my apology! And I did enjoy the conversation as well, but all things must come to an end.

E+J and Lirenel, I thought your back-and-forth was great and respectful. That's why I liked reading through it. I always enjoy discussions that make me ponder what I believe and why I believe it. Thanks for the opportunity to participate, albeit briefly! I'm impressed with the insights of each of you