Complex sentence: Gennadius was, together with his mentor, Mark of Ephesus, involved in the Council of Florence which aimed to end the schism between East and West, and had studied and written extensively on Western theology.
Split into simpler sentences: Gennadius was, together with his mentor, Mark of Ephesus, involved in the Council of Florence which aimed to end the schism between East and West. Gennadius had studied and written extensively on Western theology.