It's the figure of Jesus himself.
And it was he who lay at the heart of early Christianity's
The early centuries of Christianity were not a period
of peace and goodwill. Far from it.
They were torn apart by religious controversy about the nature
and divine essence of Jesus.
There were crucial religious issues at stake.
What was the exact relationship
between Jesus and God?
What and where had Jesus been
before he was born to Mary?
How could a perfect and indivisible God
give up part of himself to create a son?
And, so - and this was the killer question for many -
were Jesus and God made of the same substance?
Or were they just very like each other?
The mosaics here make a very strong case
for the divine status of Jesus,
as if to erode any misunderstanding
because he appears as part of a calculated scheme of images
designed to end the controversy,
telling the viewer exactly what to believe.
In perfect alignment
are three different aspects of Jesus.
The apse, there's the beardless Jesus, young,
the son of God.
The centre of the ceiling,
there's Jesus as the symbolic lamb of God,
the Jesus who's to be sacrificed on behalf of humanity.
And, at the top of the entrance arch,
there's the older, bearded,
who's about as indistinguishable
as you could get from God the Father.
So, there's a lesson here
in seeing Jesus.
And, also, particularly in that last image, a clear steer.