LOL! Here we go again on the "merry-go-round" of reformed Christianity... It gives me a headache just thinking about the circular argument that this "doctrine" relies upon.
How do YOU know who is a "true Christian"? People today claim that they are "true Christians", only to be outcast by the "remaining true Christians" who said the other guy was "never one to begin with" 10 years later! You call that a "guarantee"?
You are right that there will be some who claim the same thing as I do who will be lost. No getting around that. That's why I can't claim with Divine certainty.
Ah, NOW we are getting somewhere!
Regards
Outside of myself I DON'T know who is a true Christian. I just have educated guesses that mean nothing in the grand scheme of things. I don't "outcast" any other Christians, and I couldn't care less if others "outcast" me. I know in whom I believe. The guarantee is found in scripture and I can know if it applies to me.