This painting is part of an altarpiece, the main panel of which depicts The Virgin and Child. This section shows Saint John the Baptist; other parts of the altarpiece show Saints Peter, Philip and John the Evangelist.
John the Baptist was the prophet who foretold the coming of Jesus. He was related to Jesus through his mother Elizabeth, who was a cousin of the Virgin Mary. John lived as a hermit in the desert, preaching and baptising people in the River Jordan - it is here that Jesus came to him to be baptised and John declared him the Son of God. In this painting, he is depicted with a scroll upon which is inscribed ECCE. AGNVS. DEI ('Behold, the Lamb of God').