Clerics are devoted followers of a god who has granted them divine power. What exactly is necessary to become a cleric varies from god to god, as each rewards the qualities they deem important. Some clerics are unbridled zealots, literally frothing at the mouth with passion for their faith. Others are wise men who ponder the deep mysteries of their faith.
One thing clerics are not is idle; their god has granted them power for them to use it. Indeed, most clerics were already champions of their faith’s cause when they were empowered.