Leaving aside the matter of post hoc ergo propter hoc being a basic logical fallacy, what you are saying that two individuals can both undergo an experience of conversion, both can be convinced of their "eternal security", both can face great temptation - and the one who succumbs is therefore lost and the one who does not fall is saved. So the fact of the one's election and the fact of the other's damnation is not really knowable to either, or to the people who know them, until one falls. Do you see the difficulty? I don't see the difficulty. This "matter" of being a basic logical fallacy was addressed by the Apostle John in his first epistle. In I John 2:18-27, he says:
Dear children, this is the last hour; and as you have heard that the antichrist is coming, even now many antichrists have come. This is how we know it is the last hour. They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us. But you have an anointing from the Holy One, and all of you know the truth. I do not write to you because you do not know the truth, but because you do know it and because no lie comes from the truth. Who is the liar? It is whoever denies that Jesus is the Christ. Such a person is the antichrist—denying the Father and the Son. No one who denies the Son has the Father; whoever acknowledges the Son has the Father also. As for you, see that what you have heard from the beginning remains in you. If it does, you also will remain in the Son and in the Father. And this is what he promised us—eternal life. I am writing these things to you about those who are trying to lead you astray. As for you, the anointing you received from him remains in you, and you do not need anyone to teach you. But as his anointing teaches you about all things and as that anointing is real, not counterfeit—just as it has taught you, remain in him.
There IS a difference between someone coming to saving faith in Jesus Christ and one who "claims" he has. God tells us that one way to know for sure is that the "anointing remains" in us and the Holy Spirit is present within teaching all things. It cannot be counterfeited - at least not to God's view. Granted, there are people who become backslidden in their Christian walk, and the causes are varied, but a TRUE child of God will not remain in that state. The pastor in this article who claims to now be an atheist, has forsaken his faith - but, if he ever really had it, he will not die in that state. At some point, he WILL return home. Only God knows his heart and, just as the father of the prodigal in Jesus' parable, our heavenly Father WILL welcome him home.
My point is this, bb: unless one is willing to believe that individuals like Charles Templeton were deliberately deceiving everyone while they outwardly professed Christianity - that his twenty years as a preacher and missionary were an elaborate practical joke - then it is entirely possible to be personally convinced that you are saved, and yet not be saved.