A cross of this sort, also known as an ‘enkolpion’, could be worn as a pectoral cross, and was an attribute of bishops during the Middle Ages. Over the course of the centuries, many of these crosses were produced in such a way as to hold a secondary relic. They might contain, for example, part of a saint’s clothing, pieces of the True Cross, or hair fragments. The cross was the most popular Christian symbol in Byzantium: it offered protection to the wearer and would have been available all over the Empire. It is unusual to see a cross dedicated only to one saint, as often the figure of Christ is also displayed on one side. In this case there are three possible St John’s to whom this cross refers: St John the Evangelist (co-author of the Christian Bible), St John the Baptist (also referred to as the Forerunner, who precedes the arrival of Jesus to announce his coming) and St John Chrysostom (Archbishop of Constantinople and one of the most prolific Christian Church authors).
To find out more about the Byzantine world please see our relevant blog post: The Byzantine Empire: Art and Christianity.