Sent from Goa to Ethiopia as a missionary in 1589, Páez was held captive in Yemen for seven years, from 1590 to 1596, where he used his time to learn Arabic. He finally arrived at Massawa in 1603, and made his way to Fremona, which was the Jesuit base in that land. Unlike his predecessor, Andre de Oviedo, Paul Henze describes him as "gentle, learned, considerate of the feelings of others". When summoned to the court of the young Za Dengel, his knowledge of Amharic and Ge'ez, as well as his knowledge of Ethiopian customs impressed the sovereign so much that Za Dengel decided to convert from the Ethiopian Orthodox Tewahido Church to Catholicism -- although Páez warned him not to announce his declaration too quickly. However, when Za Dengel proclaimed changes in the observance of the Sabbath, Páez retired to Fremona, and waited out the ensuing civil war that ended with the emperor's death.
This caution benefited Páez when Susenyos assumed the throne in 1607. Sissinios invited him to his court, where the two became friends. Sissinios made a grant of land to Páez on the peninsula of Gorgora on the north side of Lake Tana, where he built a new center for his fellow Jesuits, starting with a stone church. Paez is believed to be the first European to have discovered the source of the Blue Nile on april 21.st 1618. (Sir Wallis Budge , A history of Ethiopia, p. 397.) Eventually Páez also converted Sissinios to Catholicism shortly before his own death in 1622.
In addition to translating the Roman Catechism into Ge'ez, Paez is believed to be the author of the treatise De Abyssinorum erroribus. Páez's writings are one of the few works in Portuguese about Ethiopia that have not been translated into English.