Complex sentence: Saint Gildas (c.494 or 516--c.570) is one of the best-documented figures of the church in the British Isles of the sixth century, whose renowned learning and literary style earned him the designation Gildas Sapiens (Gildas the Wise).
Split into simpler sentences: Saint Gildas (c.494 or 516--c.570) was one of the best-documented figures of the Christian church in the British Isles during the 6th century. His renowned learning and literary style earned him the designation Gildas Sapiens (Gildas the Wise).