It must have been an enormous and splendid choirbook to contain initials as large and as extensively gilded as this.

Historiated initial ‘T'(erribilis) est locus iste: hic dom[us Dei est], circa 1450
ink and gold leaf on vellum

For a historiated initial with the same iconography from an English manuscript dating to 1446, see: British Library, Arundel 109, f. 159v

The words Terribilis est locus iste: hic domus Dei est are the opening antiphon to a mass dedicated to the consecration of a church, as shown here with a bishop standing outside a cathedral with holy water.