Complex sentence: Photius refused to accept the supremacy of the pope in Eastern matters or accept the filioque clause, which the Latin delegation at his council of his consecration pressed him to accept in order to secure their support.
Split into simpler sentences: Photius refused to accept the supremacy of the pope in Eastern matters or accept the filioque clause. The Latin delegation at the council of his consecration pressed him to accept the clause in order to secure their support.