Hi NS, it's not a matter of your list of options not being "good" ones, the problem with this thread begins with the fact that your title statement is itself, a fallacy, since it assumes something that cannot happen.
Those who leave the church, who also "claim" or "believe" that they were once Christians, never were. One of the ways that we know this is, in fact, seen in their choice to leave.
1 John 2
19 They went out from us, but they didn't belong to us; for if they had belonged to us, they would have remained with us. But they left, that it might be shown that none of them belonged to us.
Some may even appear to be "super" Christians (if you will .. to themselves and/or to others in the church), only to find out at the Judgment that they were never Christians to begin with (no matter what they claimed or believed or did in this life). Again, the Bible makes this clear (this concerns the day of Judgment, Jesus is speaking):
Matthew 7
22 “Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’
23 “And then I will declare to them, ‘I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.’
I, in fact, believed myself to be a Christian all of my life. I was baptized as an infant, regularly attended and enjoyed Sunday School, was confirmed at age 12, and then attended Sunday services regularly until college. I sang in the choir, went on retreats, etc., you know, the whole 9 yards, so you can perhaps imagine how surprised I was to find out that I'd NEVER been a Christian on the day when I finally became one (2 months after my 30th birthday).
So, you may be able to find out what it feels like to move from claiming/thinking you are a Christian to claiming to be an atheist, but you cannot find out what you really want to know. A true Christian cannot become UN-born spiritually anymore than he/she can become UN-born physically ..
John 3:3,6.
Yours and His,
David
p.s. - BTW, the Lord Himself confirms this fact about true Christians .. e.g.
John 6:37-40, 44.