Although this and the preceding two scenes are not explicitly related
to any obvious hagiographic source, it is a common topos to show this
sequence; a saint preaching to/converting a pagan ruler, followed by the
latter's baptism, followed by the building of the first church or cathedral,
signifying the establishment of the new diocese/bishopric. I suspect what
is shown in these panels is therefore the foundation myth of the Bishopric
of Sens, as related in the 11th century 'Gerbertine legend'.