You cannot have a program called “Cathedral Reflections” and not include Gregorian chants.
What is Gregorian chant?
Gregorian chant is a form of sacred song in Latin (and occasionally Greek), employed within the Roman Catholic Church for centuries. From the start, Gregorian chant has had two key distinguishing features:
– It is unaccompanied, meaning that there are no musical instruments accompanying the singing
– It is monophonic, which means that there is just one melodic line followed by all the singers. This contrasts with later religious and secular music, in which the different voices (soprano, alto, etcetera) may sing different, although harmonizing, vocal lines.1

1From Classical Music, BBC Music Magazine