The Byzantine Empire was a Christian state founded by Constantine the Great, and as such, crosses were popular symbols for both official and personal use. In the ancient world, personal handwritten signatures were not used as a sole means of identification, and so most documents required a seal, whereby the sender’s insignia could be pressed into wax in order to verify the authenticity of the document.
To disocver more about Byzantine art, please visit our relevant blog post: The Byzantine Empire, Art and Christianity.