Alonso Antonio de San Martín was born in Madrid, Spain on 12 December 1642.
[2][3] He was the illegitimate son of King Felipe IV of Spain and of a young woman who served in the Buen Retiro Palace, named Tomasa María de Aldana, a native of Illescas, Toledo.
However, in the newborn's baptismal certificate, which took place in the parish of San Sebastián, are assigned as biological parents: Juan de Valdés and María Díaz.
[4] On 1 October 1675, he was selected by the King of Spain and confirmed by Pope Clement X on 16 December 1675 as Bishop of Oviedo.
[2][3] On 21 October 1681, he was appointed during the papacy of Pope Innocent XI as Bishop of Cuenca.