Complex sentence: Said to be the originator of Christianity in Egypt, Saint Mark the Evangelist headed the church after having split off from the Byzantine Church in 451, the Coptic Church retained old traditions that are unique to its history.
Split into simpler sentences: Said to be the originator of Christianity in Egypt, Saint Mark the Evangelist headed the church. After having split off from the Byzantine Church in 451, the Coptic Church retained old traditions that are unique to its history.