Such a cross, also known as an enkolpion, could have been worn as a pectoral cross, which, during the Middle Ages, was an attribute of bishops. Throughout the centuries a great number of crosses were made to hold a secondary relic in them containing pieces of saints’ clothing, pieces of the True Cross, hair fragments, and so on. Crosses offered protection to the wearer and would have been available all over the Byzantine 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 we have three possibilities for which St John can refer to: 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) or St John Chrysostom (Archbishop of Constantinople and one of the most prolific Christian Church authors).
To learn more about Byzantine crosses, please visit our relevant blog post: Enkolpion Crosses in the Byzantine Empire.