Just like they have been forced to believe that god exists, they can also be forced to believe he does not or that a different god exists. I am not saying that this would take the form of physical torture but often in constant bombardment of contrary information, much the same reason they became Christian in the first place.
I am not a fan of the brain washing tactics as shown above. I find it is more civilized to allow people the chance at discovery of truth for themselves. Once you approach the idea of gods from a neutral position, without bias, you would most likely find that a god would be an illogical conclusion given known the known facts.
Accepting that almost everyone accepts that there are around 2 billion Christians on the earth and using the definition of "Christian" from every reasonable context in which I've heard it, it seems safe to say that there exists at least one Christian that would rather deny God once than face days or weeks of torture. I'm not a fan of dismissing arguments outright, but I think it is fair to say that most arguments on my right are based on some semantic redefinition of "Christian" that, rather than using language for the sake of effective communicate with this community, redefines words for the sake of argument.
The apostles acted in unity once - And they all left Him and fled. (Mark 14:50). Peter (head of the Apostles) denied Christ 3 times. And if you look at some of the things that Stalin did to priests in order to break them I think that anybody would've cracked.
Then they started murdering and torturing pagans, out of vengeance and a desire to propagate their rhetoric and further the goals of their empire.
They slaughtered their way across Europe and lazed across the continent like a fat, bloated, sickly albatross, riddled with corruption and decay.
The world is still reeling from the oppression of the Christian dark ages.
We lost so much word history, simply because the Christians in power were too frightened of freedom and critical thought. As they should have been.
Freedom ushered in the fall of Catholicism, and rightly so. The oppression of the Church has been felt by every western nation.
Thank the gods for the enlightenment.
But speaking on the likelihood of a Christian renouncing their beliefs? Surely a true believer has renounced under pain of death, torture, or threats for fear of what they may lose.
Regardless of the likelihood or the specifics of their "salvation", it's a possibility, so YES is the way to go.
Peter denied Jesus three times under the threat of violence. Countless people have denied being Christian in the face of persecution, or any religion or philosophy for that matter, for their lives. People have also lied and denied for lesser reasons such as attaining affection or maintaining interest with someone else. Even going so far as to just work for gain.
I say it could never happen. Someone can 'claim' to be a Christian, but when it comes down to threats or torture, they will deny God. There are no threats, or tortures that would make a true Christian deny God. If you don't believe me, history is full of martyrs who refused to deny Him, no matter what was done to them.
To attempt to "force" someone to deny an entity such as the Christian God only gets a verbal "no, doesn't exist." It does nothing to change their true feelings and, even worse, actively encourages their illusion. It is a stupid and immoral tactic, used not by people in favor of free expression but by tyrants of many varieties to foster the belief that their way was the best way. It doesn't make the competing point valid or make it that the victim will really believe in the absence of their deity.
Even though lying is a sin, a true Christian is one that does three things:
1. Believes in God
2. Believes that Jesus is God's son ("Is" rather than "was")
3. Confesses all of their sins
They could easily lie about their beliefs. Any of us can do it, and as of yet devices necessary to force people to change their beliefs do not exist. To be a Christian, you do not have to attend services. To be a Christian, you do not have to give money to the poor. To be a Christian, you do not have to do missionary time.
But I ask you this:
Which is more admirable, lying or telling the truth? Which is the one that, according to the Bible, Jesus would do?
Maybe the ones that would lie do not truly believe that they will live forever.