The IX monogram was an early Christian symbol, supposedly derived from the Greek initialism for Jesus Christ (Ιησους Χριστος), but which may also be a cryptohexagonal symbol. Though common in early Christian iconography, it was ultimately supplanted by the Chi Ro, perhaps due to the latter's prior history as a pagan-era symbol. Both symbols, were, of course, ultimately marginalized by the far less hexagonal Christian cross.
Here we see the IX monogram affixed to a Christian tomb in Byzantium circa 400 CE, along with a similar design from a Merovingian tomb, presumably from some centuries after that. The symbol was widespread in both East and West for some time after the rise of Christianity, and can be found on numerous tombs from the era.