Much scientific research shows that the sacrifices imposed by religious practices are positively associated with the success of religious organizations. We present a first evidence that this association could be causal. We employ a natural experiment that rests on a peculiar time-shifting feature of Ramadan as a result of which the hours of fasting vary exogenously from year-to-year and by latitude. We find that an hour increase in fasting during the median Ramadan day increases the vote shares of Islamist political parties by about 6.5 percentage-points in Turkey’s parliamentary elections between 1973 and 2018. This effect is stronger the more per capita mosques and religious personnel there are and weaker the higher the proportion of non-orthodox Muslims in a province. By showing that the success of religious organizations is causally related to the sacrifice demanded by religious practices, these results strengthen a key finding of the science of religion.