This painting is part of an altarpiece, the main panel of which depicts The Virgin and Child. This section shows Saint John the Evangelist; other parts of the altarpiece show Saints Peter, Philip, and John the Baptist.
John, with his brother James, was one of the first and youngest apostles. He is considered to have been Jesus's favourite disciple, and it is he to whom Jesus entrusted the care of his mother after his death. He is usually portrayed as a young, beardless man.