Complex sentence: Part I begins with the prophecy of the Messiah and his virgin birth by several prophets, namely Isaiah, and his birth, shows the annunciation to the shepherds as the only scene from a Gospel in the oratorio, and reflects on the Messiah's deeds.
Split into simpler sentences: Part I begins with the prophecy of the Messiah and his virgin birth by several prophets, namely Isaiah. His birth is still rendered in words by Isaiah, followed by the annunciation to the shepherds as the only scene from a Gospel in the oratorio, and reflections on the Messiah's deeds.