Never, but you are.
The reason that you believe that a Christian can become an atheist, is based solely on your own personal experience, and has absolutely nothing to do with the truth.
It is impossible to know God, or to be known by God and then to unknow God or to be unknown by God. You need a an elementary lesson in the meanings of words.
A person can divorce their spouse, but they cannot unknow them.
Besides, as I've told you already, Christianity is not a marriage. We are sons of God, and members of His Body. You cannot leave His Body, because it's His Body, not yours.
Your anti-Christ doctrine is easy to expose.
Analagies are always limited and can often be twisted to support opposite views.
I am a theist, not an atheist, so it is not just anecdote.
Knowing God is different than knowing about God. Reciprocal relationship involves more than a knowledge or belief of concepts. Jesus knows every sinner, but is not in right relationship with all men. I knew about God and Christianity before I trusted Him and knew Him intimately in surrender and love.
LH uses similar know/unknow arguments, but does not distinguish intimate relationship from knowing facts without relationship. JWs make the mistake when they add words to a verse in John to make eternal life about taking in knowledge about God (reading their material) vs KNOWING HIM, the true condition for life (know vs know about the president, get it?).
If a person divorces their spouse and remarries, they do not remain married to two people. They have pictures and memories, but no intimate relationship anymore. Divorce and marriage is an imagery used about Israel, God people, and the Church (marriage in Eph. 5).
You are making a metaphor about the Church as Body (cf. building, army, family, etc.) into a wooden literalism (Jesus' physical body does not have fingers falling off and leaving). Likewise, spiritual rebirth and adoptive sonship is based on relationship, not irreversible metaphysics. It is analagous to human birth and relations, not identical in every sense (so don't press it just to match your theology, because I can also press it to match mine and undermine yours).
The issues include free will, mind, faith vs unbelief, etc., not just assumptions that make physical and spiritual reality identical, not analagous.
Not a bad answer, if I don't say so myself.