There's a more serious discussion that could be had about this, but I'm not seeing it.
On the one hand you have the tradition of taking seriously the issue of proclaiming your choice of religion, with a tradition of martyrdom - you should not mind being killed because you say you're a Christian. That's consistent also with the commandment not to tell a lie, but I don't think that's really the issue or that there isn't an 'exception' against a greater evil - like lying to a hostage taker threatening hostages - though.
On the other hand, I think there's a widespread view agreeing with lying - why get killed by some radical terrorist over not saying what he demands you say? It's a bit like when you read a statement saying things you don't agree with at gunpoint for a camera they use as propaganda. A few might say 'you shouldn't do that', but a large majority are fine with saying whatever they want to not get shot. It's understood it's under duress.
There is a real gap between these two views, and our strong preference for the latter does confront us with what to do with the celebrated legacy of martyrs who did not.
Is it different if 'the government' (not the Obama government, crazy tea party, but like Ancient Romans or Maoist China) demands you take an 'official' position?
How about the long history of England where Catholics and Protestants alternated power and killed the other side? I don't recall the history of how much just 'changing sides' was an option, was chosen, how many were not killed that way, but my impression is many were killed either without that choice or for not taking it, and that just saying you switched wasn't that well thought of.
Where is the line drawn on what pressure is ok? What if they threaten to fire you? What if they threaten to fine you ten dollars? What if they offer you 30 silver pieces to convert?
Part of the history of these issues is that the people demanding the conversions know it's easy for people to lie, and they can make more demands. You say you convert - and they order you to kill your friend who refused to. Do you? It's not the same thing, but I recall hearing of the horrible choice given to some Jews in concentration camps, to assist with the processing and avoid being killed that way. But it's a similar 'what's the right thing to do' issue.
One thing that changes the issue is that on the one extreme, if you imagine that they *needed* the Jews to do it, and the Jews united in their refusal to cooperate, and it had a big effect on preventing the killing, that's easy to praise. But on the other, if refusing was simply something one person could do and they were immediately replaced and it didn't make any difference to the killing, far less easy to say they should do that.
It's an easy call for most to say 'say what the terrorists want'. The situation changes when it's not just that, but people are forced to live by the other rules - denounce your former religion is a small escalation, practice the new religion for years is another escalation, but go to war to fight for the new side and kill your own side, betray people on your own side so they're killed, actively work to support the war machine of the new side, are a few examples where this becomes a lot harder question than 'say you convert'.
For a bit of a possible answer, I think the general principle of coercion/duress excusing your doing what's demanded makes sense, though that line has to be found where it's too little coercion to justify it. It gets harder if you are ordered to work in a factory making bombs that will kill many of your people. It raises a question, if they are relying on your people's labor for that, are you better off not cooperating and saving other lives? And against that's a harder question if it's 'just you' and the impact is questionable.
But not that easy to accept doing things that will kill a lot of innocent people, either.
Put in the situation you are put in their military and told 'shoot those members of your own people or you'll get shot' - is either answer clearly the right one? From the view if you're the person your fellow follower is asked to shoot, you might hope he refuses, if it might help you.
The underlying issue though is how this duress is abusive to people - and yet effective in a lot of situations, and rarely questioned by the side doing it.
If an invading country can hire or pressure one group of the enemy to fight another, does anyone really say 'we shouldn't do that, it's not right'?
Another example - German or Japanese occupied countries in WWII, take the French resistance. We love to praise the resistance, but was it really that wrong for the people under threat to cooperate? Isn't condemning that inconsistent with the thing we're defending here of converting when Isis demands it? And yet it's hard not to praise the people who took great risks to fight the occupiers - and many will call those who cooperated under duress 'traitors'.
Isn't it funny how we try to simply an issue like that by just deciding a position such as 'anyone who cooperated was wrong', when I think it's a lot less clear than that?
But what about that moral issue of an occupied people forced to supply a war machine that will kill many others? What if that makes the difference in the evil side 'winning the war' and millions in the future living under tyranny because of it, does the duress still justify helping them? How much does it make a difference what the impact of cooperating is, is there a line where it'll be more harmful changes what's right?
And it's one thing for us to armchair hindsight look at this, but it recalls the statement by an American financier in the 1930's:
'I can always pay half of the poor to kill the other half.'
It's easy to condemn that paying, but if you are picking which half of the poor to join in that situation... and what use is the condemnation, if the situation allows it?
History is filled with that situation, and the guy who pays gaining power and writing the history.
We like to praise those who resist, yet we also love to make their stories come out where they actually defeat the tyrant, which doesn't usually seem to be an option.
We praise the family who housed Anne Frank, but if they were all shot for doing so, wouldn't condemning them for not housing her somewhat contradict the 'convert' answer?
As much as martyrdom has a history, I strongly lean towards the 'say what they want under duress' position. But it does get a lot harder with other choices.
How can we better prevent this sort of use of power for ruthless control and forcing people to be in those situations?
It's easy to condemn if the 'other guys' do it - what if it's your side?
It's not really analogous, but I think of when one group recruits spies and agents from the others, the difference between how quickly we condemn ones against us as horrible traitors, and yet have no issue with the other side doing the same thing and serving us as our spies and agents. For one example I think of the pressure - the duress - Israel can put on Palestinians to be their spies, the harm they cause their own people, and the fury with which Palestinians treat anyone who does it.
That comparison between the furious hate of such a traitor, with Israeli citizens presumably having no problem with doing it, seems relevant.
That goes from 'convert' at the point of a gun, to 'serve' the other side.
This doesn't even get into the issue of when a 'traitor' feels they are 'doing the right thing'. We sure don't view any German who tried to assassinate Hitler badly. I'm thinking of the cases of Russians in the Cold War who admired things like the respect for freedom of the west and helped us as spies - but what if an American betrayed our government for doing what they think is wrong, such as the illegal war under Reagan in Nicaragua, or they felt giving the Soviets nuclear secrets would prevent our launching nuclear war?
This is about duress, and that is an ugly issue.