Complex sentence: The Nativity of Jesus--the circumstances surrounding the birth of Jesus of Nazareth, who was the Jewish Messiah according to Christian belief--is described in the gospels of Luke and Matthew.
Split into simpler sentences: The Nativity of Jesus (or simply The Nativity) is the circumstances surrounding the birth of Jesus of Nazareth, who was the Jewish Messiah according to Christian belief. It is described in the gospels of Luke and Matthew.