March 31, which instead forced all Jews in Spain to convert or be expelled within four months. Those who converted became known as conversos (converts). This move, along with the progressive erosion of other guarantees provided by the surrender treaty, raised tensions and fears within the remaining Muslim community during the 1490s. Many of the city's affluent Muslims and its traditional ruling classes emigrated to North Africa in the early years after the conquest, but these early emigrants numbered only a few thousand, with the rest of the population unable to afford leaving. By 1499, Cardinal Francisco Jiménez de Cisneros grew frustrated with the slow pace of the efforts of the first archbishop of Granada, Hernando de Talavera, to convert non-Christians and undertook a program of forced baptisms, creating the converso class for Muslims and Jews. Cisneros's new strategy, which was a direct violation of the terms of the treaty, provoked the Rebellion of the Alpujarras (1499–1501) centered in the rural Alpujarras region southeast of the city. The rebellion lasted until 1500 in Granada and continued until 1501 in the Alpujarras. Responding to the rebellion of 1501, the Crown of Castile rescinded the Treaty of Granada, and mandated that Granada's Muslims convert or emigrate. Many of the remaining Muslim elites subsequently emigrated to North Africa. The majority of the Granada's mudéjares converted (becoming the so-called moriscos or Moorish) so that they could stay. Both populations of converts were subject to persecution, execution, or exile, and each had cells that practiced their original religion in secrecy (the so-called marranos in the case of the conversos accused of the charge of crypto-Judaism). Over the course of the 16th century, Granada took on an ever more Catholic and Castilian character, as immigrants arrived from other regions of Castile, lured by the promise of economic opportunities in the newly conquered city. At the time of the city's surrender in 1492 it had a population of 50,000 which included only a handful of Christians (mostly captives), but by 1561 (the year of the first royal census of the city) the population was composed of over 30,000 Christian immigrants and approximately 15,000 moriscos. After 1492 the city's first churches had been installed in some converted mosques. The vast majority of the city's remaining mosques were subsequently converted into churches during and after the mass conversions of 1500. In 1531, Charles V founded the University of Granada