Leaders of the Eastern and Western churches both

believed they had complete power.

had power over European kings.

believed a schism was unnecessary.

allowed priests to get married.

Among the choices, only "believed they had complete power" is the feasible choice.

Leaders of the Eastern and Western Churches both believe they had complete power.

Since both churches represent God. They believe that they too have complete power over the people who believe in God. They were held in high regard due to their religious position and beliefs.