Religion in the Balkans during the Middle Ages was, of course, important in the lives of peastant and nobles alike. However, the rules were not so stringent that religions of convenience could not be practiced. For example, the Bogomils were extreme ascetics whose practices appealed to the peasant classes (who naturally had very few material possessions, and who therefore found the idea of reaching salvation through denial of worldly goods a winning proposition).
The Crypto-Christians, on the other hand, were Christians who outwardly converted to Islam in order to keep their worldly goods and gain favor with authority once the Ottomans expanded into the Southeastern European territory.
In fact, the Bogomils found it rather easy to become Crypto-Christians, due to the general feelings of disdain they got from the Orthodox church. (Couldn't they then be called Crypto-Bogomils?) Religion of convenience indeed.