Complex sentence: The diocese was formed in 1996, when the Canadian and American branches of the Reformed Episcopal Church were reunited and was composed of parishes extending back to the founding of the REC in the 1970s.
Split into simpler sentences: The diocese was formed in 1996 when the Canadian and American branches of the Reformed Episcopal Church were reunited. It was, at the time, composed of parishes extending back to the founding of the REC in the 1970s.