Priests weren't (and, I suppose , still aren't) expected to do heavy labor or manage a household, and they had to be literate to perform the Mass and all of the various sacraments. They were all scholars of religion and philosophy by training, and had the free time to persue other studies if they desired. That said, being a priest isn't just a job, it's a 24/7 commitment. Catholic priests had to give up all lands and any possibility of marriage, and especially in the middle ages the performance of various religious ceremonies took up a LOT of time. They also had to actually manage churches as institutions, and churches themselves could own lots of land and have tenants and whole economies under their purview.