In general, citizenship in the way we identify with it today arose in Ancient Greece, where citizens were part of the state and had a say in the affairs and direction of the state.
It promoted the idea that governments should have limited power ! :) (apex)
The significance of Christ's presence during the crowning of emperors in Byzantine art relates to the notion that this individuals are pre-ordained by god with certain powers and thus are ordained to be leaders of their community. By placing Jesus in this pictures it signals that these emperors were chosen by god and Jesus to rule thus granting them religious legitimacy to their rule.