Complex sentence: The few remaining Catholics of Idstein were not permitted to hold services until 1806, when they were granted to use the "Schlosskapelle" (Palace Chapel) until 1888, when they could move to their own church.
Split into simpler sentences: The few remaining Catholics of Idstein were not permitted to hold services until 1806. Then they were granted to use the "Schlosskapelle" (Palace Chapel) until 1888, when they could move to their own church.