John Gonzales de Castrillo was born at Sahagun, Léon in Spain. He was the first of seven children born to wealthy parents who did not have any children for the first sixteen years of their marriage. His parents John and Sancia brought their children up well and taught them how to love God.

He studied at the Fagondez abbey run by Benedictine monks of his town Sahagun. When he grew up John became a priest. He could have lived a very comfortable life in the cathedral parish or in other wealthy parishes. But John preferred the poverty and simple lifestyle that Jesus had lived. Father John chose to keep charge only of a small chapel. There he celebrated Mass, preached and taught catechism.

Father John realized that he needed to know theology (religious studies) better. He started attending classes at the great Catholic University of Salamanca. After four years of hard study, he became a famous preacher.

Nine years later after he got seriously ill, John joined a community of Augustinian friars. They were very impressed by the way he practiced the Christian virtues. He was obedient to his superiors and humble, too. He also continued his preaching.

His beautiful homilies or sermons changed the hearts of the people of Salamanca who had been quarreling violently among themselves. Often young noblemen fought each other in revenge. St. John was able to end many of these bitter fights and convinced people to forgive one another.

He was not afraid to correct evils, even when the evildoers were powerful people who could take revenge. Once he corrected a duke at Alba de Tormes for the way he was making the poor people suffer. In anger, the duke sent two of his men to kill St. John. The two men approached Father John and found him so calm and kind. Both men were sorry they had come to harm this holy priest and asked him to forgive them.

Then the duke became sick and was healed through the prayers of St. John. He repented of his sins and recovered fully. St. John could read hearts of people at confession. People traveled great distances to seek his advice and ask for spiritual guidance.

It was the graces he received from prayer and from the Mass that gave St. John his special power as a preacher. He celebrated the Mass with great devotion and often saw the Host surrounded by light. He sometimes saw Jesus in body at the moment of consecration.

St. John was known to work many miracles. Once in Salamanca a small child fell into a well. The people did all they could but could not save the child and they quickly sent for Father John. When he got to the well, he laid his waistband on the stone wall of the well, and prayed that the waters return the child. The well water rose to ground level, floating the child to safety.

St. John of Sahagun died on June 11, 1479.