Who was the famous medieval composer who wrote the first complete musical setting of the Roman Catholic Church?

Answer 1

There were no distinguished composers for the first musical pieces to be sung in medieval cathedrals. However, they (as well as any other music before it that we know of) were not documented until Pope Gregory charged his monks with devising a way to write lyrics and sheet music for the songs to preserve them. They are now known as either "Gregorian Chant" or "Plainsong", and are still preformed in European cathedrals today.