Rodney Stark, a sociologist of religion, states that "Saint Thomas Aquinas deduced that slavery was a sin, and a series of popes upheld his position, beginning in 1435..." Nevertheless, for several decades spanning the late 15th and early 16th centuries, several popes explicitly endorsed the slavery of non-Christians. In 1452, as the Ottoman Empire was besieging Constantinople, the Byzantine Emperor Constantine XI asked for help from Pope Nicholas V. In response, the pope authorized King Alfonso V of Portugal to "attack, conquer, and subjugate Saracens, pagans and other enemies of Christ wherever they may be found...", in the bull Dum Diversas (18 June 1452). Rather than putting pressure on the Ottomans, however, the bull approved increased competition in West Africa, by Portuguese traders with Muslim-operated trans-Saharan trading caravans, including the highly profitable so-called Trans Saharan slave trade that had taken place for several centuries. In 1454, Castilians also became involved in trading in various goods in West Africa, and were attacked by Portuguese warships. Enrique IV of Castile threatened war and Afonso V appealed to the Pope to support monopolies on the part of any particular Christian state able to open trade with a particular, non-Christian region or countries. A papal bull, Romanus Pontifex, issued on January 8, 1455, conferred upon Portugal exclusive trading rights to areas between Morocco and the East Indies, with the rights to conquer and convert the inhabitants. A significant concession given by Nicholas in a brief issued to Alfonso V in 1454 extended the rights granted to existing territories to all those that might be taken in the future. and sanctioned the purchase of slaves from "the infidel" (i.e. non-Christian): "many Guineamen and other negroes, taken by force, and some by barter of unprohibited articles, or by other lawful contract of purchase, have been ... converted to the Catholic faith, and it is hoped ... that ... such progress be continued ... [and] either those peoples will be converted to the faith or at least the souls of many of them will be gained for Christ." By dealing directly with local leaders and traders, the Portuguese government sought to control trade with West Africa. In effect, the two bulls issued by Nicholas V conceded to subjects of Christian countries the religious authority to acquire as many slaves from non-Christians as they wished, by force or trade. These concessions were confirmed by bulls issued by Pope