According to Islam, one's faith and belief is more valuable than his/her life. There have even been cases in which people sacrificed their lives for their belief. However, it is noteworthy to mention that the issue of belief is something internal. Hence, an individual may face a situation such as persuasion or reluctance of some people in which he/she is not able to express his true faith and belief. In fact it is so that in that same situation, openly expressing his true belief will jeopardize his life. According to both the Holy Quran and to one's wisdom, when placed in a dangerous situation one should remain silent in order to protect his/her life. Even if it was necessary one should express against his own beliefs. (Let it be emphasized again that faith and belief is more sacred than one's life, however expressing a belief is different from having a belief.)
Such action, which is actually dissimulation of one's religion under duress or in the face of threatening damage, is called Taqiyyah. Moreover, Allah (SWT) has made a distinction in the holy Quran between the non-believers and those who do Taqiyya. Several verses in the Holy Quran express the fact that Taqiyyah is permissible and the people who act upon it are Muslims. (1)
An example of Taqiyya in the Holy Quran can be seen in the story about the believing man who was from the people of Pharaoh. Though the Holy Quran mentions that this man hid his faith, Allah (SWT) verifies him as a believer. (2) Taqiyya like many other Islamic rules however, is not applicable at all times. Rather it is permissible only in few situations. For example if Taqiyya of one person causes other people in the society to fall astray and accept that fallacy as truth, then doing Taqiyya is not permissible in that situation at all. In other words one of the underlying purposes of Taqiyya is maintaining the religion. Naturally when it comes to issues such as Jihad (struggle), enjoining what is good and forbidding what is evil, and similar obligatory rules, Taqiyyah is not permissible.