Celebrated for his social work he has been called the Vincent de Paul of Japan and at some point became Master of the Order of Preachers in the country.

His example quickly became an inspiration to later missionaries to Japan from the Order of Preachers such as canonised saints Giordano Ansalone, Domingo Ibáñez de Erquicia, Guillaume Courtet, and Lorenzo Ruiz.

Presumably an eldest son, he gave up his inheritance to enter the Order of Friars Preachers at the Priory of San Pablo in Valladolid at an early age.

Alfonso encouraged Yoshiaki to return to the faith sometime between his appointment as head of the Dominicans in Japan and before the Daimyo's death but to no avail.

On May 22, 1617, two priests, the Jesuit Blessed João Baptista Machado and a Franciscan companion, were martyred at the young Daimyo's headquarters Kushima Castle in Ōmura.

An account from a 1901 compendium of Dominican Hagiographies describes Alfonso's death thusly: "After dragging him from one desert island to another, in order to find some spot where his execution might take place unknown to the Christians, the soldiers at length struck of his head as he knelt in prayer, holding his rosary and a blessed candle in one hand, and a wooden cross in the other.