Sanctuary
Ceiling
The blue shields form part of the Santuary ceiling and are found above the High Altar.
In the Latin-speaking Christianity of medieval Western Europe (and so among Catholics and many Protestants today), the most common Christogram became "IHS" or "IHC", denoting the first three letters of the Greek name of Jesus (ΙΗΣΟΥΣ), iota-eta-sigma, or ΙΗΣ.
The Greek letter iota is represented by I, the eta by H, while the Greek letter sigma is either in its lunate form, represented by C, or its final form, represented by S. Because the Latin-alphabet letters I and J were not systematically distinguished until the 17th century, "JHS" and "JHC" are equivalent to "IHS" and "IHC".
XPI (chi-rho-iota) comes simply from the first three letters of the Greek spelling ΧΡΙΣΤΟΣ (Christos) meaning Christ.