Páez was born in 1564 in the village of Olmeda de la Cebolla (now Olmeda de las Fuentes), near Madrid, which until August of that year had been a hamlet of the city of Alcalá de Henares. He was born sixteen years before the union of the Spanish and the Portuguese crowns (1580-1640); that union would determine the span of Páez's missionary activity. In his youth, he studied at the Jesuit college in Belmonte, Cuenca. He was long thought to have then proceeded to study in the University of Coimbra, but recent studies have shown that he did his higher studies at the University of Alcalá. Around that time, he entered the Society of Jesus and was later ordained a priest.