What was the preferred two-dimensional medium for didactic, christian art during the middle ages, particularly in the byzantine empire?

The preferred two-dimensional medium for didactic, Christian art during the Middle Ages, particularly in the Byzantine Empire was known as mosaic.
As you already probably known, a mosaic is created by assembling small pieces of colored glass, stone, or other material to create some sort of a picture. These were didactic in the Middle Ages because they showed scenes of a good and honorable life of saints.