If there is a solid reason for you to make christians think you are converted to christianity, than that's allowed.
What a solid reason might be life threat, extreme life threatening situation makes it allowed for you to say/do anything to save your life, but this does not mean one will stop trying to escape such situation/region to dar-us-salaam.
Whoever disbelieves in [i.e. denies] Allaah after his belief, except
for one who is forced [to renounce his religion] while his heart is
secure in faith. But those who [willingly] open their hearts to
disbelief, upon them is wrath from Allaah, and for them is a great
Now in the case you of some undercover work like spying etc, if it is absolute necessary to do such job, there are some conditions to follow, with which it is allowed.
take every precaution for yourselves. (Translation- Mohsin Khan) Quran(4:102)
Conditions include this activity should be for the benefit of muslim ummah, not any individual or group. There is no other way to do this job other than showing oneself as non-muslim. And that the person will not do any major sin doing this job, and is a strong enough believer that he will not be deceived by all things going around in that time. In short it is allowed with some conditions.
For details see Muslims working as spies on enemies of Islam.
Allah knows best