What is Crypto-Christianity?: Crypto-Christianity was called "double faith" by the Southeastern Slavic peoples. It was the outward conversion to Islam while privately practicing the Christian faith. Publicly, Crypto-Christians were Muslims, and, in a world overtaken by Ottomans, this offered them some benefits.
Privately, however, these same people held Catholic or Orthodox Christian beliefs. The gradual expansion of the Ottomans during the 14th century caused the people of Southeastern Europe to convert under duress or for preservation of wealth and power.
Crypto-Christianity in Bulgaria: Because the Bulgarians were defeated in war by the Ottomans, they faced forced conversion to Islam. Certain populations converted to Islam first. Any members of the nobility who remained converted quickly to this new religion in order to maintain possessions and authority. The Bogomils also found conversion relatively easy due to being ostracized for their heretical religious beliefs and practices.
Crypto-Christianity in Albania: Albanians, because they were not conquered by the Ottomans in wartime, were first permitted to practice any faith they wished. After battles broke out, however, pressure was put on them to convert from Christianity to Islam.
This pressure may not have been the sole purpose for Albanians' willing conversion. The Ottomans in power looked upon converts to Islam favorably, so, like the Bulgarian nobility, Albanians probably became Crypto-Christians due to the desire to take advantage of fewer taxes and other material gain.
Crypto-Christians could also be found in Serbia, though there is less evidence of their existence than of the Crypto-Christians of Bulgaria and Albania. Some pockets of Crypto-Christians were still practicing at the turn of the 20th century.
Image of Pantocrator Church in Nessabar, Bulgaria
References
Skendi, Stavros. "Crypto-Christianity in the Balkan Area under the Ottomans." Slavic Review, Vol. 26, No.2. 1967. 227-246.