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Calvinist theology doesn't allow that an atheist who claims to have once been a Christian ever actually was one. Catholic (and Arminian Protestant) theology does hold that folks can fall away from true faith. I never believed otherwise (thus in my critique of your deconversion I never denied that you were a Christian). I was an Arminian Protestant and am now a Catholic.
The "perseverance" / eternal security position is a minority viewpoint in historic and present-day Christianity. Catholics deny this; so do Orthodox (that's already some 1.4 billion Christians). So do most pentecostals, Lutherans, Methodists, Anglicans and lots of non-denominational groups. It is mainly the Presbyterians, Reformed, Baptists, and smaller groups related to them theologically, who hold this (greatly unbiblical) position.
Before Protestantism arose in the 16th century, Christians virtually unanimously agreed that falling away was possible.
So argue against it; you are right when you do that, but be sure to note that this is only one position within Christianity, and a minority one at that.
Of course the downside of the opinion that you were a Christian, would be that you, therefore, rejected Christianity and the first hand experience you had with it (and with God), and are now (by definition) an apostate. The ones who claim you never were a Christian cannot really say that. They'd have to say you were a "wolf-in-sheep's clothing." So it's either that or an apostate.
Either way, the future doesn't look too bright for you, from where we sit.
But my theology and approach tries to adopt a middle way as much as possible: objectively you are an apostate, but subjectively there may be many reasons (mitigating circumstances) why you left (or that influenced your decision) that would cause God to exercise mercy on the last day. That is my hope.
The key would be if you truly knew Christianity was the truth and rejected it. That is very serious. Only God knows if you had and have full and sufficient knowledge or not.
If you didn't, and didn't now, there is hope that you may be saved, because you are not directly rejecting something you know to be true, but rather, mistakenly believing a falsehood that you sincerely believe to be true. In Catholic theology, this is a very large factor.
In any event, our job as Christians (of whatever type) is to convince you to embrace Jesus and Christianity again (or for the first time, so say the Calvinists, etc.). That is obviously far better than to be an atheist, from our vantage point.
You know this; it isn't like I'm saying anything new. But what we believe on this affects how we approach people. Those who think you are unregenerate and never-saved will tend to be (but don't necessarily have to be) more rude and presumptuous about your soul and ultimate destination.
I make no claims on either your sincerity or the state of your soul or moral character. None whatsoever. I simply critiqued the reasons you gave for your deconversion. I don't see why that would be insulting to anyone (as it is merely entering into the arena of competing ideas), yet John Loftus blew a gasket when I examined his story.
Go figure, huh?
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"I make no claims on either your sincerity or the state of your soul or moral character. None whatsoever."
I should clarify that I assume in charity that you are sincere. I rarely deny or question anyone's sincerity (having often had mine denied, by dialogical opponents).
The same applies to soul-states and character. I would only arrive at a negative appraisal after repeated, compelling evidence to the contrary.
Some of you may also be interested in knowing that John Calvin actually denied that anyone can know for sure that they or anyone else is of the elect. I wrote a paper about it.
So it is not even right (according to the origin of their own system) for a Calvinist to flatly deny that a person is in the elect. We simply don't know, because we don't know the future. God could save you on the last day of your life. We don't know what could possibly change in the end.
Practically speaking, it works out the same: the Calvinist would simply say that last day was the only time you were "saved." All Christians would say that you were always of the elect if in fact you were saved on the last day of your life, because God knows all things. But we couldn't know the ultimate destiny of your soul.
All anyone can do is go by a person's profession of faith and what they do (because almost all Christians agree that some fruit should be apparent in a true Christian's life).