A true Holy Spirit filled Christian that has the Revelation of Jesus Christ? No, it's impossible. You can never become an true atheist then. You are sealed by the Holy Spirit. You might go in some state or form of denial but not all the way, the seed of the Word will never leave that person. Such a person cannot help but to believe from the heart.

Anyone that has fallen to the state of being an true atheist or even joining some other religion never had what they thought they had. Even if they sacrificed all and lived everyday with all zeal and effort. They never saw what they thought they saw. Even if they were sincere like Cain bringing his offering. But like him, their sacrifice was incorrect without revelation.

No, its impossible. Scripture says “A son of God cannot sin for the seed remaineth in him” - 1 John 3:9. Sin means disbelief or unbelief. In other words, they failed to recognize the Truth.

Now for the strange thing. A person might go crazy, loose his mind and don't know who he is anymore but if you were once sealed by the Holy Ghost then God will remain in your soul no matter what. You can never be lost. Because insanity is in the mental realm. God lives in the soul realm.

If this gospel is hid, it's hid to them that are lost. If you are a christian turned atheist you are typed by Israel that died in the wilderness before reaching the promised land. Yet they carried out the feasts, partook in the things of God and even walked with the prophet and saw the miracles.

Jesus said they were all dead. In other words, lost. They served God with the mind but did not have God in their hearts and when difficulties came there was no anchor in the soul.

But if you are talking of the nominal steriotype christian? For sure. I think today's churches are probably to blame for most christian turned atheists. I hardly blame them from turning away from the churches.