I want to hit it again, because it is such a critical point--the "reasoning to" versus "reasoning from" God situation.
It also pertains to this clarifying verse: "They went out from us, but they were not of us; for if they had been of us, they would have continued with us: but they went out , that they might be made manifest that they all are not of us." --1 John 2:19
When people say, "I use to be a Christian, but ____________" you know that "they all are not of us."
For if you are reasoning FROM God, there is no way that God is going to lead you away from Himself. Instead what is the case is that the "new" unbeliever never really believed in the first place. They were always an unbeliever. While professing to be a follower of Jesus Christ, instead the person was still keeping their self as the judge and arbiter. They were yet to keep the very first commandment "you shall have no other gods before Me."
If you are a disciple of the Lord then HE becomes the Lord of your reasoning; for a man cannot serve two masters. And in such cases, the unbeliever first and foremost relies on their own perception, opinion, intellect and weighs revelation against that. It is impossible to have the Spirit of Truth indwelling and then go from knowledge of the truth into error and lack of knowledge. So when someone says, "I use to be a Christian" . . . you can rest assured that they never were in the first place.
If you start from God, rather than from fallen self, He will lead to greater and greater understanding and experience of truth, as He IS the truth. Those who claim to have once believed only believed in an idol that they had constructed, which was a lie from the start. Then they lose belief in that idol and make a new one to follow . . . until it also lets them down . . . and they make another, and another and so on. "Everyone is senseless and without knowledge; every goldsmith is shamed by his idols. The images he makes are a fraud; they have no breath in them." --Jer. 10:14
Those with the most staying power usually end up as "existentialists" or in some other form of nihilistic reductionism. They collect at the bottom of the pool of utter absurdity.
But it is not quite right to say that they never knew God exists in the first place. Consciously, they refuse to assent to His existence and Lordship, because deep down, they DO know He is real, but they "suppress the truth in unrighteousness." "For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened. Professing to be wise, they became fools,--" --Rom. 1:21-22
Rebellion is at the core of it.
But oh how they like to carry on in their willful ignorance--in that rebellion!
"Fools have no interest in understanding; they only want to air their own opinions."
Here are some more applicable zingers from Proverbs 18:
"One who isolates himself pursues selfish desires, he quarrels against all sound wisdom. . . . A fool’s lips bring strife, and his mouth calls for blows. A fool’s mouth is his ruin, and his lips are the snare of his soul. Before destruction the heart of man is haughty, but humility goes before honor.
He who gives an answer before he hears, it is folly and shame to him."