Complex sentence: Maternus, also known as St. Maternus, was the first Christian bishop of Cologne, Germany (in the 4th century) who commissioned a Roman temple where the Cologne Cathedral would later be built.
Split into simpler sentences: Maternus, also known as St. Maternus, was the first bishop of Cologne, Germany, in the 4th century. He commissioned a Roman temple where the Cologne Cathedral would later be built.