The kingdoms of Portugal and Castile had been jockeying for position and possession of colonial territories along the African coast for more than a century prior to Columbus' "discovery" of lands in the western seas. On the theory that the Pope was an arbitrator between nations, each kingdom had sought and obtained Papal bulls at various times to bolster its claims, on the grounds that its activities served to spread Christianity.
The bull Romanus Pontifex is an important example of the Papacy's claim to spiritual lordship of the whole world and of its role in regulating relations among Christian princes and between Christians and "unbelievers" ("heathens" and "infidels"). This bull became the basis for Portugal's later claim to lands in the "new world," a claim which was countered by Castile and the bull Inter caetera in 1493.
An English translation of Romanus Pontifex is reproduced below, as published in European Treaties bearing on the History of the United States and its Dependencies to 1648, Frances Gardiner Davenport, editor, Carnegie Institution of Washington, 1917, Washington, D.C., at pp. 20-26. The original text in Latin is in the same volume, at pp. 13-20.