Apart from the effect of a lengthy period under Ottoman domination, many of the subject population were periodically and forcefully converted to Islam as a result of a deliberate move by the Ottomans as part of a policy of ensuring the loyalty of the population against a potential Venetian invasion. However, Islam was spread by force in the areas under the control of the Ottoman Sultan through devşirme and jizya. Rather Arnold explains Islam's spread by quoting 17th-century author Johannes Scheffler who stated: