Complex sentence: The parish is part of the Archdiocese of Seattle, and served as that diocese's cathedral from the diocese's establishment in 1850 until 1907, when the episcopal see was transferred to Seattle.
Split into simpler sentences: The parish is part of the Archdiocese of Seattle and held an important role in that diocese's founding. The church served as the diocesesan cathedral from the diocese's establishment in 1850 until 1907, when the episcopal see was transferred to Seattle.