The Crucifix of Mogiła in the Sanctuary of the Holy Cross has attracted throngs of pilgrims for centuries. Yet the beginnings of the its public veneration cannot be ascertained. According to tradition, it was fished out from the Vistula River in the 12th century and brought to the monks as a gift. It is, however, possible that the wooden figure of Crucified Christ was a gift of King Casimir the Great (Kazimierz Wielki) for the completion of renovation works conducted in the monastery by Abbot John (1333–56), or else a special request for forgiveness. The story of Casimir the Great arriving to take a look at the restored church is still told in the Cistercian Abbey. The monarch was brought by the abbot to the image of the Crucified Lord. Looking with interest at Jesus, the king asked whether any miracles take place here. The abbot confirmed that yes they do, yet it would be the greatest miracle if his majesty returned from his wrongful ways, like Saul turned from an enemy of Christ to his faithful disciple. The king left without a word. Soon, however, he made amends with Bishop Bodzanta, who had cursed him for his immoral ways.

The Mogiła Crucifix is peculiar and enjoys the steady devotion of the congregation, although its original form has changed. It is the only element of church furnishing to have been saved from a fire of the church in the 15th century. Only Christ’s hair and loincloth (perizoma) burnt down. The sculpted cloth was then replaced with a band of embroidered velvet that covers the figure nearly down to the knees, and Christ received a wig of real hair. After the reconstruction of the church in the 16th century, the crucifix was moved to a side chapel which to this day is known as the Chapel of the Holy Cross.

The numerous graces and healings made the mother of Adam Chmielowski (later St Brother Albert), who came here on a pilgrimage in the intention of her son’s health when he fell gravely ill. Little Adam, dressed for the ceremony in a monk’s habit, was offered before the miraculous image of the Crucified Christ.

The sanctuary in Mogiła also has relics of the True Cross. It was first mentioned in the first half of the 20th century, yet as the authenticity of the relic was challenged, Father Bogumił Salwiński began entreating the Apostolic See for a new one. As a result, a large section of the Holy Cross relic from Santa Croce Basilica in Rome was brought in 1961.