Gain more believers, as to become as powerful as possible.
Uriel, among many other top ranking angels in Jehovah, became dissatisfied with only the Hebrews. Uriel thought they could claim more power if they lessened the restrictions and gained more followers. They left Jehovah and formed Yahweh. Uriel and his followers devised they could use what the Hebrew people have added to rules of Jehovah, a messiah. They chose a human couple, because they thought it would be easier to just train a human from birth, so the human would grow with power naturally. Gabriel spoke to Mary and Joseph, the couple, to warn them to convince everyone that it was a virgin birth, but to go ahead and make a child before marriage.
Once that was set up and the child was born, Jesus, the angels awakened his aura and began to secretly training. All the while making knew laws for people to follow. They devised people would gather in dedicated buildings and release their power in the building and the angels would just stop to collect. To have a constant source of power they would have to spread this new religion as far as they could.