This is a relatively recent acquisition for the Cleveland Museum of Art. Christ is shown on sitting on the right hand of the Father, which is a reference to the Nicene Creed. The dove in the middle represents the Holy Spirit.
In the West, we think that the Roman Empire fell in 476 AD, which then brought about the early middle ages, aka the Dark Ages. But in the East, they would say that the Roman Empire fell on May 29th, 1453, because the Roman Empire lasted for almost another thousand years in the East, with Constantinople as its capital. In 1453, the Ottomans breached the gates, ending the Roman Empire, and changing Constantinople to Istanbul.