The Solving of a Mystery in the Lindisfarne Gospels

Some time ago I wrote a piece about the St Matthew of the Lindisfarne Gospels. Here. The image is painted by Br Eadrith and was done in the 8th century.

At the time I was unsure as to who the figure peeping from behind the curtain might be:

Now, a year later a reader, Rev Dan Bodine wrote to me saying:

The most obvious candidate in my view is St. Luke. The gospels of Matthew and Luke share much of the same material. The painting would therefore accuse Luke as listening in on the Divine Word as Matthew receives it. (Who wouldn't)

So, mystery solved...unless anyone has any more suggestions...