Complex sentence: Babai the Great was the inofficial head of the Assyrian Church of the East at the beginning of the 7th century, revived the Assyrian monastic movement, and formulated the Christology of his church.
Split into simpler sentences: Babai the Great was one of the pillars of the Assyrian Church of the East. At the beginning of the 7th century he was the unofficial head of his Church, revived the Assyrian monastic movement, and formulated the Christology of his church.